Twilio Programmable Voice API + MCP vs Voximplant

Twilio Programmable Voice API + MCP

Twilio APIs SLA at 99.95 per cent for every paying customer, with 10 per cent credits. US outbound at $0.014 a minute, plus $0.0044 for Media Streams or $0.07 for ConversationRelay.

Voximplant

Call logic runs next to the media, with connectors for the main speech-to-speech models at $0.004 a minute. Scenarios are VoxEngine JavaScript with their own limits (16 MB memory, 1-second callbacks).

Twilio Programmable Voice API + MCP

1. Use <Connect><Stream> for two-way audio. It blocks further TwiML until the websocket closes
2. Use <Connect><ConversationRelay> when you only want to handle text, and set interruptible for barge-in
3. Trial calls reach only verified numbers in your sign-up country
4. Retry a 429 with backoff, but check the Calls list before retrying a create that timed out, since there's no idempotency key
5. Validate X-Twilio-Signature on every webhook and websocket upgrade

Voximplant

1. Upload a VoxEngine scenario and a rule first, then start it with StartScenarios and pass the number in script_custom_data
2. Use a service-account JWT rather than api_key in the URL
3. Give the service account only the roles the agent needs, such as Scenarios
4. Ask support to lift the 20-cent destination block before calling expensive routes