A voice AI API for phone agents

The Stemvo API lets your product take and make phone calls with an AI agent. Create agents and calls over REST, receive every call event by webhook and connect your own SIP trunk, all for €0.15 a minute with speech, AI model and voice included.

Get an API key See a call in code

Start a call in one request

Pick an agent and a number. The agent calls, talks and sends the result to your webhook when the call ends.

REST API with keys per project
Webhooks for every call event
Transcript, recording and summary per call
Same agents as the dashboard

Start an outbound callcurl

# create the call
curl https://api.stemvo.eu/v1/calls \
  -H "Authorization: Bearer $STEMVO_KEY" \
  -d '{
    "agent_id": "agt_dental_nl",
    "to": "+31 20 555 0134",
    "language": "nl"
  }'

# 201 Created
{ "id": "call_8f2c", "status": "ringing" }

# webhook when it ends
{ "event": "call.ended", "id": "call_8f2c",
  "duration_s": 74, "outcome": "booked",
  "summary": "Check-up booked, Thu 10:30" }

What you can build with the voice AI API

The phone side of your product, without running the voice stack yourself.

Inbound agents

Point a number at an agent and it answers, books into your system through your tools and reports back.

Outbound calls

Reminders, confirmations and follow-ups started from your product with one API call.

Tools and functions

Let the agent call your endpoints during the call to check availability, look up orders or create bookings.

Your own phone lines

Bring your SIP trunk or carrier, or buy local numbers through the API.

Many languages

Set the language per agent or per call. The agent can also follow the caller’s language.

Accounts for your customers

Create a workspace per customer and read usage per workspace for your own billing.

One price per minute, whatever the call needs

Speech, AI model, voice and platform are one number. Your unit cost stays the same as prompts grow.

€0.15per minute, all included

€0monthly platform fee

At costphone numbers

60 minfree to build and test

See full pricing

Developer questions

Is there a voice AI API?

Yes. The Stemvo API lets you create agents, start calls, read transcripts and recordings, and receive call events by webhook. Everything in the dashboard is also in the API.

How much does the voice AI API cost?

€0.15 per minute of calls, with speech recognition, the AI model and the voice included. There is no monthly platform fee, and phone numbers are billed at cost.

Can I bring my own SIP trunk?

Yes. Connect your SIP trunk or carrier, or buy local numbers through the API.

Can the agent call my own API during a call?

Yes. Define tools with your endpoints, and the agent calls them during the conversation, for example to check free slots or look up an order.

Can I move agents from another voice platform?

Yes. Copy the prompt and tools into a new agent over the API or in the dashboard, then point your number or SIP trunk at it.

Give your product a phone line.

60 free minutes to build and test. No card needed.

Get an API key