Major Takeaways
Who should pick Retell AI over Synthflow?
Teams with engineering resources building custom voice products that need low-level control over latency, LLM choice, and telephony integrations.
Who should pick Synthflow over Retell AI?
Operations and contact-centre teams that need to deploy AI phone agents quickly without code, using visual workflows and pre-built templates.
How do their pricing models differ?
Retell AI charges per-minute usage plus separate LLM and telephony costs; Synthflow bundles platform access, minutes, and telephony into tiered monthly plans.
If you care about X, pick Y
| If you care about | Pick | Why |
|---|---|---|
| No-code deployment | Synthflow | Visual workflow builder; no engineering required to launch agents |
| API-level control and customization | Retell AI | Full SDK access; custom LLM integration; granular latency tuning |
| Transparent all-in pricing | Synthflow | Published monthly tiers include minutes and telephony; no surprise line-item bills |
| LLM and voice provider flexibility | Retell AI | Bring your own OpenAI, Anthropic, or custom model; swap TTS providers |
| Outbound campaign management | Synthflow | Built-in campaign scheduler and list management; Retell AI requires external orchestration |
| Developer-first documentation | Retell AI | API docs, SDKs, and WebSocket references for custom builds |
How we evaluated
This comparison draws on vendor documentation from Retell AI's developer portal and Synthflow's knowledge base, published pricing pages as of October 2026, and third-party reviews from G2, Capterra, and TrustRadius. We evaluated build experience, telephony flexibility, pricing transparency, LLM and voice customization, compliance documentation, and inbound versus outbound capabilities. No test calls were placed; all performance claims are vendor-stated. Pricing figures reflect list rates at time of writing and exclude volume discounts or enterprise contracts. Last updated: October 2026.
What each platform does
Retell AI
Retell AI is an API-first voice AI platform for developers building custom phone agents.
Teams integrate via REST API or WebSocket SDK to control conversation flow, LLM selection, interruption handling, and telephony routing. The platform supports inbound and outbound calls, function calling during conversations, and bring-your-own carrier or SIP trunk configurations. Retell AI is used by engineering teams building voice products for customer support, sales qualification, and appointment booking. The vendor does not publish customer counts or total call volume processed.
Synthflow
Synthflow is a no-code AI voice agent builder for operations teams automating inbound and outbound calls without engineering resources. Users build agents in a visual workflow editor, selecting from pre-built templates for appointment booking, lead qualification, and customer support.
The platform includes bundled telephony, campaign management for outbound calling, and one-click integrations with CRMs and calendars. Synthflow targets contact centres, real estate agencies, home services businesses, and SMB sales teams. The vendor reports serving customers across 40+ countries but does not publish total agent counts.
Feature comparison
| Capability | Retell AI | Synthflow |
|---|---|---|
| Build model | API-first; SDK and WebSocket | No-code visual builder; templates |
| Inbound calls | ✓ | ✓ |
| Outbound calls / campaigns | ✓ (requires external orchestration) | ✓ (built-in campaign scheduler) |
| Telephony | Bring-your-own carrier; SIP trunk; Twilio integration | Bundled phone numbers in 40+ countries; bring-your-own optional |
| LLM choice | Bring-your-own (OpenAI, Anthropic, custom) | Vendor-hosted (model not specified publicly) |
| Voice / TTS provider choice | Swap providers (ElevenLabs, Deepgram, Azure); voice cloning supported | Limited to platform voices; cloning on higher tiers |
| Languages | Not specified in public docs | 20+ languages (vendor-stated) |
| Interruption handling | Configurable turn-taking thresholds via API | Platform-managed; settings not exposed |
| Warm transfer / human handoff | ✓ (via function call) | ✓ (workflow action) |
| Function calling / tool use | ✓ (custom functions during call) | ✓ (pre-built integrations; limited custom) |
| CRM and calendar integrations | API-based; requires custom build | One-click (HubSpot, Salesforce, Google Calendar, Calendly) |
| Call recording, transcripts, analytics | ✓ (API access to recordings and logs) | ✓ (dashboard analytics; export available) |
| Compliance statements | SOC 2 Type II (vendor-stated) | GDPR-compliant (vendor-stated); SOC 2 not listed |
| Concurrency limits | Not publicly listed; scales per account | Tier-based (50–500 concurrent calls depending on plan) |
Pricing and cost per minute
Retell AI pricing
Retell AI does not publish a public pricing page with per-minute rates. Prospective customers request a quote after signing up for API access.
The platform charges per-minute usage billed separately from LLM inference and telephony costs. Based on third-party reports, per-minute platform fees are estimated in the range of $0.05–$0.15 per minute, excluding LLM tokens (billed by OpenAI, Anthropic, or the customer's provider) and telephony (billed by Twilio or the customer's carrier). No minimum spend or contract term is publicly listed. Enterprise customers negotiate custom rates.
| Component | Estimated cost | Notes |
|---|---|---|
| Platform fee | $0.05–$0.15/min (estimated) | Not publicly listed; request quote |
| LLM inference | Variable (customer's provider) | Billed separately by OpenAI, Anthropic, etc. |
| Telephony | Variable (customer's carrier) | Twilio or bring-your-own SIP trunk |
| Speech-to-text / TTS | Variable (customer's provider) | Deepgram, ElevenLabs, Azure, etc. |
Source: Third-party estimates; Retell AI documentation does not list per-minute pricing.
Synthflow pricing
Synthflow publishes tiered monthly plans on its pricing page. Plans include platform access, bundled minutes, and telephony (phone numbers in 40+ countries). Overage minutes are billed at a per-minute rate specified in each tier.
The vendor lists four tiers as of October 2026: Starter ($99/month, 500 minutes included), Professional ($299/month, 2,000 minutes included), Business ($799/month, 6,000 minutes included), and Enterprise (custom pricing, unlimited minutes). All tiers include CRM integrations, call recording, and analytics. Voice cloning and advanced workflow options are available on Professional and above.
| Plan | Monthly cost | Included minutes | Overage rate | Notes | |---|---|---|---| | Starter | $99 | 500 | $0.20/min | Basic workflows; single phone number | | Professional | $299 | 2,000 | $0.15/min | Voice cloning; advanced integrations | | Business | $799 | 6,000 | $0.12/min | Priority support; 5 phone numbers | | Enterprise | Custom | Unlimited | N/A | Dedicated account manager; SLA |
Source: Synthflow pricing page, October 2026.
What a month of calls might cost
To estimate monthly spend, calculate: (average minutes per call) × (total calls per month) × (all-in per-minute rate) + platform fees + telephony base costs + overage charges.
For Retell AI, the all-in per-minute rate is the sum of the platform fee, LLM inference cost (typically $0.02–$0.10 per minute depending on model and prompt length), speech-to-text ($0.01–$0.03 per minute), text-to-speech ($0.02–$0.08 per minute), and telephony ($0.01–$0.02 per minute for Twilio).
An illustrative example: if a team runs 1,000 calls per month averaging 3 minutes each, with a platform fee of $0.10/min, LLM cost of $0.05/min, STT/TTS of $0.05/min combined, and telephony of $0.02/min, the total is (3 × 1,000 × $0.22) = $660/month. Actual costs vary widely based on LLM choice, voice provider, and call complexity.
For Synthflow, if the same 1,000 calls (3,000 minutes total) fit within the Professional tier's 2,000 included minutes, the team pays $299 base + (1,000 overage minutes × $0.15) = $449/month. If usage fits entirely within the included minutes, the cost is the flat monthly fee.
What users say
Public review coverage for Retell AI was limited at the time of writing. The platform's developer-first positioning means it appears less often on consumer review sites like G2 or Capterra, where end-user operations teams typically leave feedback. Available reviews on G2 note the platform's flexibility and API documentation quality, though some users report the need for engineering resources to deploy and maintain agents.
Synthflow
Synthflow has broader review coverage on G2 and Capterra. Users highlight the no-code builder and fast deployment. Common themes include ease of use for non-technical teams and the quality of pre-built templates.
Some reviewers note limitations in customization compared to code-based platforms, and occasional difficulties with complex multi-step workflows.
Strengths and gaps
What Retell AI does well
API-first architecture. Full SDK and WebSocket access gives developers granular control over conversation flow, latency tuning, and telephony routing. Teams building custom voice products can integrate Retell AI into existing systems without platform constraints.
LLM and voice provider flexibility. Bring-your-own model support (OpenAI, Anthropic, custom fine-tuned models) and swappable TTS providers (ElevenLabs, Deepgram, Azure) let teams optimize for cost, quality, or compliance requirements.
Function calling during conversations. The platform supports custom function execution mid-call (CRM lookups, booking confirmations, data retrieval), which enables dynamic workflows without pre-scripted paths.
SOC 2 Type II compliance. Retell AI states SOC 2 Type II certification, which addresses security and availability controls for teams in regulated industries.
Where Retell AI has gaps
Retell AI does not publish per-minute pricing or all-in cost estimates, which creates procurement friction for teams comparing total cost of ownership.
The platform requires engineering resources to deploy and maintain agents; operations teams without developer support may find the API-first model a barrier. Outbound campaign management (list uploads, scheduling, retry logic) is not built into the platform and requires external orchestration. Language support is not specified in public documentation, which may limit international deployments.
What Synthflow does well
No-code visual builder. Operations teams can build and deploy AI phone agents without writing code, using drag-and-drop workflows and pre-built templates for common use cases (appointment booking, lead qualification, customer support).
Transparent bundled pricing. Published monthly tiers include platform access, minutes, and telephony, which simplifies budgeting and eliminates surprise line-item bills for LLM inference or carrier fees.
Built-in outbound campaign management. The platform includes list upload, call scheduling, and retry logic for outbound campaigns, which removes the need for external orchestration tools.
One-click CRM integrations. Pre-built connectors for HubSpot, Salesforce, Google Calendar, and Calendly reduce setup time compared to API-based integrations.
Where Synthflow has gaps
Synthflow does not expose low-level controls for latency tuning, interruption handling, or LLM selection, which limits customization for teams with specific technical requirements.
The platform's vendor-hosted LLM model is not publicly specified, which may concern teams with data residency or compliance constraints. SOC 2 certification is not listed in public documentation, though the vendor states GDPR compliance. Advanced workflow customization is reportedly limited compared to code-based platforms, based on third-party reviews.
Which use cases fit each
| Use case | Stronger fit | Why |
|---|---|---|
| Inbound lead qualification and speed-to-lead | Either | Both handle inbound calls; Synthflow faster to deploy, Retell AI more customizable |
| Appointment booking | Synthflow | Pre-built calendar integrations; no-code workflow setup |
| After-hours reception | Synthflow | Template-based setup; bundled telephony simplifies deployment |
| Outbound reminders and confirmations | Synthflow | Built-in campaign scheduler; list management included |
| Outbound sales calling | Either | Synthflow for ops-led teams; Retell AI for custom qualification logic |
| Customer-support deflection | Retell AI | Custom function calling enables complex CRM lookups and escalation logic |
| High-volume contact-centre overflow | Synthflow | Tier-based concurrency limits and bundled pricing scale predictably |
| Developer building a voice product | Retell AI | API-first; full control over LLM, TTS, and telephony stack |
Pick Retell AI if you...
- Have engineering resources and need API-level control over conversation flow, latency, and telephony routing.
- Want to bring your own LLM (OpenAI, Anthropic, custom fine-tuned models) or swap TTS providers for cost or quality optimization.
- Are building a custom voice product that requires function calling, dynamic workflows, or integration into existing systems.
- Need SOC 2 Type II compliance and prefer to manage data residency and model selection directly.
Pick Synthflow if you...
- Need to deploy AI phone agents quickly without engineering resources, using visual workflows and pre-built templates.
- Want transparent all-in pricing with bundled minutes and telephony to simplify budgeting and avoid surprise line-item bills.
- Are running outbound campaigns and need built-in list management, scheduling, and retry logic without external orchestration.
- Prefer one-click CRM and calendar integrations (HubSpot, Salesforce, Google Calendar) over API-based custom builds.
Note: if the calls you're automating are inbound sales conversations that need to sit alongside an outbound AI SDR in one platform, teams in that position typically also evaluate 11x's Julian.
Verdict
Retell AI is the stronger fit for engineering teams building custom voice products that need API-level control, LLM flexibility, and the ability to integrate into existing systems.
The platform's SDK, WebSocket access, and function-calling capabilities give developers the tools to optimize latency, customize conversation logic, and manage data residency. Teams with the resources to handle API integration and external orchestration for outbound campaigns will find Retell AI's flexibility worth the engineering overhead. The lack of published per-minute pricing and the need for separate LLM and telephony billing create procurement friction, but the platform's technical depth compensates for teams with specific requirements.
Synthflow is the better starting point for operations and contact-centre teams that need to deploy AI phone agents quickly without code.
The no-code builder, pre-built templates, and bundled pricing (minutes and telephony included) remove technical barriers and simplify budgeting. Built-in outbound campaign management and one-click CRM integrations reduce setup time compared to API-based platforms. The trade-off is less customization: teams that need granular control over LLM selection, latency tuning, or complex multi-step workflows may find Synthflow's platform-managed approach limiting.
Practical next steps: build the same short test flow on both platforms (a simple inbound qualification script or outbound reminder call). Check the all-in per-minute rate at your expected volume, including LLM inference and telephony for Retell AI, and overage rates for Synthflow. Confirm compliance documentation (SOC 2, HIPAA, GDPR) directly with each vendor if your use case requires it. If your team has engineering capacity and needs deep customization, start with Retell AI. If you need to launch fast without code, start with Synthflow.
Frequently asked questions
Is Retell AI cheaper than Synthflow?
It depends on call volume and technical stack. Retell AI's per-minute platform fee is estimated at $0.05–$0.15 per minute, but teams pay separately for LLM inference, speech-to-text, text-to-speech, and telephony, which can add $0.05–$0.20 per minute depending on provider choices. Synthflow's bundled plans include minutes and telephony, with overage rates of $0.12–$0.20 per minute. For low-volume use cases (under 2,000 minutes per month), Synthflow's $299 Professional tier may be cheaper all-in. For high-volume deployments where teams can optimize LLM and TTS costs, Retell AI's unbundled model may cost less per minute.
Which is easier to build with if I can't code?
Synthflow. The platform's visual workflow builder and pre-built templates let non-technical teams deploy AI phone agents without writing code. Retell AI is API-first and requires engineering resources to integrate, configure, and maintain agents.
Can I bring my own phone numbers or carrier to each?
Yes, both platforms support bring-your-own carrier configurations. Retell AI integrates with Twilio or custom SIP trunks, giving teams full control over telephony routing. Synthflow includes bundled phone numbers in 40+ countries but also supports bring-your-own numbers as an optional configuration.
Which handles outbound calling campaigns better?
Synthflow. The platform includes built-in campaign management (list upload, call scheduling, retry logic) as part of the core product. Retell AI supports outbound calls via API but does not include campaign orchestration; teams need to build or integrate external tools for list management and scheduling.
Are Retell AI and Synthflow HIPAA compliant?
Retell AI states SOC 2 Type II compliance but does not list HIPAA certification in public documentation. Synthflow states GDPR compliance but does not list SOC 2 or HIPAA certifications publicly. Teams in healthcare or other regulated industries should request compliance documentation (Business Associate Agreements, audit reports) directly from each vendor before deployment.
Can either transfer a call to a human?
Yes. Retell AI supports warm transfer via function calling, which lets the agent hand off to a human mid-conversation. Synthflow includes warm transfer as a workflow action in the visual builder. Both platforms require integration with a telephony system (Twilio, SIP trunk, or the platform's bundled carrier) to route the transfer.
What do users complain about most?
Public review volume for Retell AI is limited; the recurring themes below are drawn from the available G2 / Trustpilot / TrustRadius coverage. For Retell AI, users with limited engineering resources report difficulty deploying and maintaining agents without developer support. For Synthflow, reviewers note limitations in customization for complex workflows and occasional difficulties with multi-step logic that falls outside pre-built templates.
How does 11x's Julian compare to Retell AI and Synthflow?
Julian is 11x's AI phone agent for inbound speed-to-lead and qualification, sold as part of the 11x platform alongside Alice (outbound AI SDR). It's a packaged sales agent rather than a general-purpose voice-building platform, so teams building custom voice products for customer support, appointment booking, or non-sales use cases typically compare it less directly. Julian is designed for revenue teams that want inbound voice and outbound email/LinkedIn in one system, not for developers building custom agents or operations teams automating general call workflows.
What's the main difference in deployment speed?
Synthflow is faster to deploy for non-technical teams. The no-code builder and pre-built templates let operations teams launch agents in hours or days. Retell AI requires API integration, which typically takes weeks depending on engineering capacity and the complexity of the voice product being built.
Which platform is better for multi-language deployments?
Synthflow states support for 20+ languages in public documentation. Retell AI does not specify language support publicly, so teams deploying in non-English markets should confirm language availability directly with the vendor before committing.
Last updated: October 2026.
