Retell AI vs Synthflow 2026: Billing, Pricing, Integrations
Updated 2026-06-13 · Reviewed by Jordan M. Reyes
Retell AI and Synthflow are both voice-agent platforms, but they target different buyers. Retell AI is a developer API with extensive webhook control and pure per-minute billing. Synthflow is a no-code dashboard platform with bundled monthly plans. This comparison goes deep on billing mechanics, LLM rate cards, and integration coverage.
Side-by-Side Overview
| Feature | Retell AI | Synthflow AI |
|---|---|---|
| Target user | Developer / agency | Non-developer / SMB |
| Setup method | API + code | Dashboard (no-code) |
| Billing model | Pure per-minute (pay-as-you-go) | Monthly plan + overage |
| Billing minimum | 10-second per connected call | Per-minute rounding |
| Voicemail billing | Flat 5-second rate | Standard rate |
| Webhook events | Extensive (call-start, end, transcript, DTMF, transfer) | Limited (call-end, basic) |
| LLM selection | OpenAI, Anthropic, Groq, others | Proprietary + limited selection |
| Native integrations | Webhook/API-based | HubSpot, Zapier, Cal.com |
| Starting price | ~$0.07\u2013$0.15/min | ~$29/month plan |
Billing Mechanics in Detail
Retell AI Billing
- Per minute, rate varies by LLM and TTS voice selected.
- Minimum per call: 10 seconds. A 3-second call is billed as 10 seconds.
- Voicemail detection: flat 5-second rate regardless of actual voicemail length.
- No monthly floor on pay-as-you-go tier.
Synthflow AI Billing
- Monthly plan with a set number of included minutes.
- Per-minute overage rate applies once included minutes are exhausted.
- Voicemail billed at standard per-minute rate (no flat voicemail discount).
- Per-minute rounding\u2014less favorable for short calls.
LLM Rate Cards
| LLM Tier | Retell AI (approx.) | Synthflow (approx.) |
|---|---|---|
| Lightweight (GPT-4o Mini equiv.) | ~$0.07/min | Included in plan |
| Mid-tier (GPT-4o equiv.) | ~$0.10/min | Included in plan (limited) |
| Frontier (Claude 3.5 Sonnet equiv.) | ~$0.15/min | Not available |
| Custom/self-hosted LLM | Supported (BYOLLM) | Not supported |
Rates are approximate and change frequently. Check each vendor\u2019s pricing page for current rates.
Integration Coverage
Synthflow wins on out-of-the-box native integrations: HubSpot CRM sync, Zapier, and Cal.com/Calendly configured in the dashboard without code.
Retell AI wins on depth: webhook events fire on call-start, call-end, transcript-ready, DTMF input, and transfer events. Developers can build sophisticated CRM sync, real-time coaching, and custom analytics pipelines.
When to Choose Each
- Choose Retell AI if: You have a developer, need custom call flows, want per-minute billing with voicemail flat-rate, or need frontier LLM access.
- Choose Synthflow if: You have no developer, need quick setup with native CRM integrations, or prefer predictable monthly billing at lower volumes.
Frequently Asked Questions
What is the main difference between Retell AI and Synthflow?
Retell AI is a developer-first API platform requiring code to configure; Synthflow is a no-code dashboard platform for non-developers. Retell AI offers more granular control and webhook events; Synthflow offers faster setup for simple use cases.
Which is cheaper: Retell AI or Synthflow?
At low volumes (under ~200 calls/month), Synthflow’s bundled monthly plan may be more predictable. At high volumes, Retell AI’s pure per-minute pricing (no monthly floor on pay-as-you-go) is typically cheaper. Build a model with your actual call volume and average duration to compare.
How does Retell AI handle voicemail?
Retell AI charges a flat five-second billing rate when a call hits voicemail and the agent detects it. This is lower than the standard per-minute rate and is a cost control feature for outbound campaigns where many calls go unanswered.
Does Synthflow have a free trial?
Synthflow typically offers a limited free tier or trial period. Check the current Synthflow pricing page for the latest offer—plans and trial terms change frequently in this market.
Which platform is better for outbound calling campaigns?
Both support outbound calling. Retell AI’s voicemail flat-rate billing and per-second webhook granularity make it more cost-efficient for high-volume outbound. Synthflow’s dashboard-configured campaigns suit non-technical teams running lower-volume outbound.
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