Issue

Receiving Error 1700 "Missing to_number in recording participant info" on POST /v3/Transcriptions when transcribing dual-channel recordings with a WebRTC Voice SDK Client leg, even when explicitly providing two participants in the request payload.

Product

Programmable Voice

Cause

Error 1700 occurs when Twilio's Intelligence service attempts to infer caller and recipient metadata (such as phone numbers or channel identities) directly from the underlying call recording, but cannot resolve a required address such as when one leg of the call is a WebRTC Voice SDK Client (client:identity) that lacks a standard PSTN phone number.

Resolution

When passing a sourceId (Recording SID starting with RE...) to the POST /v3/Transcriptions endpoint, the API attempts to resolve participant metadata from the underlying call record. If a WebRTC Client leg is involved, metadata resolution fails to extract a phone number, triggering Error 1700.

To bypass automatic metadata-based address resolution for WebRTC or non-PSTN recordings, use the mediaUrl parameter instead of sourceId in your POST /v3/Transcriptions request payload. Providing the raw recording URL (mediaUrl) alongside explicit participant objects allows you to manually define participant types, names, and addresses without relying on call metadata resolution.

Additional Information 

For more details on configuring transcriptions and defining participant metadata manually, refer to the Twilio Voice Intelligence Documentation.