“Should I build my voice agent on Vapi or Twilio ConversationRelay?”
Direct Answer
Pick Vapi for speed and a managed orchestration layer, and Twilio ConversationRelay for maximum control if you already run on Twilio and have engineers. Both need a language model and real build effort, so a managed AI receptionist is faster if you only need calls answered.
01Vapi vs Twilio ConversationRelay: the short answer
Choose Vapi when you want a working voice agent fast and are happy to pay a platform fee for orchestration, dashboards and ready-made integrations. Choose Twilio ConversationRelay when you already run on Twilio, have engineers, and want full control of the language model, the logic and the data path. Neither is a finished AI receptionist: both are building blocks. If you are also weighing carriers, our guide to Twilio alternatives for AI voice agents covers the telephony side.
02What each one actually is
Twilio ConversationRelay connects a Twilio voice call to your application over a WebSocket. Twilio handles speech-to-text and text-to-speech, turn detection and caller interruptions, plus automatic language detection; your server receives the transcript, calls the language model of your choice and sends text back to be spoken. Twilio’s own benchmarks report a median latency under half a second, though your model and hosting add to that. You bring the model and the app.
Vapi is a voice agent platform that sits above telephony, speech and model providers. You configure an assistant, pick providers for transcription, model and voice, attach tools for actions like booking, and Vapi orchestrates the live call. You can use your own provider keys and pay those providers directly.
03Cost: what you pay for in each
| Cost line | Vapi | ConversationRelay |
|---|---|---|
| Orchestration | Platform fee, about $0.05 per minute per third-party breakdowns | ConversationRelay per-minute charge from Twilio |
| Telephony | Twilio, Telnyx, Vonage or imported numbers, billed by the carrier | Twilio voice charges |
| Speech-to-text | Provider rate, or bundled | Handled by Twilio |
| Language model | Your provider, per token | Your provider, per token, plus your hosting |
| Text-to-speech | Provider rate, per character or minute | Handled by Twilio, or your own TTS |
| Engineering time | Low to medium | Medium to high |
Rates change often, so confirm current prices on Twilio’s ConversationRelay page and Vapi’s pricing page before you model costs. For the wider picture, see our breakdown of per-minute vs flat-rate voice agent pricing.
04Control, latency and engineering effort
ConversationRelay gives you the most control: your server decides every response, you own the logs, and you can run custom retrieval, guardrails or routing. The price is engineering effort and responsibility for uptime. Vapi trades some of that control for speed, since assistants, tools and call flows are configured rather than coded. On latency, both depend heavily on the language model and voice you choose, so test with your real prompts instead of trusting a headline number.
05Integrations: what plugs into each
With Vapi, integrations are mostly tools and webhooks: calendars such as Google Calendar and Calendly, CRMs such as HubSpot, and automation platforms such as Zapier, Make and n8n, alongside provider choices for speech, models and voices. With ConversationRelay, you integrate whatever your own server can call: any CRM, booking system or database with an API, which is flexible but means you build and maintain each connection. For a side-by-side of voice platforms, see Vapi vs Retell.
06How to choose
Pick Vapi if you need a prototype this week, have a small team and want dashboards and call analytics included. Pick ConversationRelay if you are already a Twilio shop, need strict control of data and logic, or plan high volumes where owning the stack pays off. Pick neither if you simply need calls answered: a managed receptionist avoids the build entirely. Our complete guide to AI voice agents and chatbots explains how to decide between building and buying.
FAQs
Is Twilio ConversationRelay cheaper than Vapi?
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Which is better for a small business that just wants calls answered?
10+ years building growth systems for SaaS, fintech, healthcare and Web3. Ex-Head of Marketing at LCX — scaled 10K → 150K users and $50M+ raised across 12 token sales. Builds voice agents, automation and AI-search systems hands-on across every vertical Growth100X serves.