Retell AI
Low-latency voice AI SDK for building phone agents with custom LLM backends, natural interruption handling, and batch calling - $12M Series A.
Retell AI is a developer SDK for building production voice AI agents that handle real phone calls with human-level conversational quality. Unlike platforms with proprietary AI backends, Retell lets developers connect any LLM - GPT-4o, Claude, Gemini, or custom models - as the reasoning engine while Retell handles all the real-time speech complexity: low-latency streaming TTS, speech recognition, turn-taking detection, and interruption handling. Founded in 2023 by Evie Wang and Evan Liu, Retell raised a $12M Series A in 2024 and serves use cases in customer service, scheduling, healthcare outreach, and sales. Pricing is $0.11 per minute with new accounts receiving $10 in free credits to start.
Key Features
- Bring-your-own-LLM architecture - connect GPT-4o, Claude, Gemini, or custom models as the AI backend
- Under 800ms end-to-end latency from speech input to voice response for natural conversation pacing
- Turn-taking and interruption detection that handles overlapping speech naturally
- Concurrent batch calling - launch thousands of simultaneous outbound calls from a single API call
- Call recording and full transcript with speaker labels for every conversation
- Webhook events for call start, end, transcript updates, and custom agent milestones
- Pre-built voice agent templates for appointment scheduling, lead qualification, and customer support
Use Cases
- Developers building AI customer service agents that handle inbound support calls autonomously
- Healthcare companies automating appointment reminder and patient follow-up calls at scale
- Sales teams running AI outbound prospecting calls to qualify leads before human handoff
- Enterprises needing custom LLM reasoning in a voice interface rather than a locked-in AI backend
Pros
- Bring-your-own-LLM is the key differentiator - teams are not locked into a single AI provider for reasoning
- Sub-800ms latency produces conversational pacing indistinguishable from human response times
- Batch calling API enables large outbound campaigns without managing telephony infrastructure
Cons
- No free tier beyond initial $10 credit - sustained development costs accumulate at $0.11 per minute
- Bring-your-own-LLM requires more engineering setup than turnkey solutions like Synthflow
- Phone number provisioning and TCPA compliance for outbound calling adds regulatory complexity
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