Voice providers
Credentials and supported features for the voice providers Simulation connects to
Simulation places voice calls through a connected provider. Vapi, Retell, and Bland.ai are the three native providers, chosen when you set up a voice agent definition. Others covers any agent you can reach by phone.
Vapi
Connecting a voice agent to Vapi needs the API key and the assistant ID from your Vapi account, both entered when you connect your agent. If the agent takes inbound calls, it also needs a contact number of 10 to 12 digits.
Retell
Connecting a voice agent to Retell needs the API key and the assistant ID from your Retell account, entered the same way as Vapi. Inbound calling needs the same 10 to 12 digit contact number.
Note
Retell agents don’t support tool call evaluation. The “Enable tool call evaluation” toggle on a run has no effect on Retell-backed calls; evaluation covers the conversation only.
Bland.ai
Bland.ai appears as Bland.ai in the provider dropdown and authenticates with an API key, the same as Vapi and Retell. Two things work differently:
- The Assistant ID field takes a Conversational Pathway ID. Bland has no separate assistant object, so open the pathway your agent runs and copy its ID into Assistant ID
- A contact number is always required, not just for inbound. Bland has no web connector, so every simulated call is placed over the phone. For inbound tests, use a Bland number attached to that pathway
Note
Bland records a single combined audio track rather than separate caller and agent channels. An eval whose input is mapped to the stereo, assistant, or customer recording resolves empty on a Bland call, so map whole-conversation evals to call.voice_recording instead. See Edit a run’s evals for where that mapping lives.
Because Bland’s API key is sent in a different header format from the other providers, switching an existing agent definition away from Bland clears the stored key. Re-enter it for the new provider.
What each provider supports
| Capability | Vapi | Retell | Bland.ai |
|---|---|---|---|
| Voice calls | Yes | Yes | Yes |
| Call recordings | Yes | Yes | Yes, one combined track |
| Call transcripts | Yes | Yes | Yes |
| Compare with baseline (replay) | Yes, the original call’s configuration comes across | Transcript only | Transcript only |
| Tool call evaluation | Yes | No | No |
| Contact number | Inbound only | Inbound only | Always required |
Recordings and transcripts show up in the call detail view the same way regardless of provider. All three support “Compare with baseline” on a call, which lines its transcript up against the reference call. What differs is how much of the original comes across, and Replay is where that difference is explained.
Limits
Voice calls are capped at 30 minutes on any provider; a call in progress is ended automatically once it hits that mark. See Call metrics for how duration, cost and talk time are reported once a call completes.
Questions & Discussion