Guides
SIP
When the voice agent and simple forwarding aren't enough and you want to connect a number to your own softphone or phone system (PBX) - this page is a step-by-step guide.
1Inbound calls to your SIP server
Set the number's routing to mode: "sip" and give it your sipUri - from there the call goes straight to you, not to our agent or forwarding:
curl -X PATCH "https://volai.cz/v1/numbers/+420601234567" \
-H "Authorization: Bearer vk_YOUR_KEY" \
-H "Content-Type: application/json" \
-d '{"routing": {"mode": "sip", "sipUri": "sip:reception@your-server.com"}}'Only the inbound minute is billed - 0.50 CZK/min (~EUR 0.02), no extra agent - the full request shape and the other routing modes are in the REST API reference.
2SIP credentials for a number
Fetch the line's credentials with a single call:
curl "https://volai.cz/v1/numbers/+420601234567/sip" \
-H "Authorization: Bearer vk_YOUR_KEY"{
"server": "sip.volai.cz",
"username": "123456",
"password": "a1b2c3d4e5f6"
}Setup in a softphone (Zoiper, Linphone)
Both apps ask for essentially the same thing, just under different names - look for "Add account" / "SIP account":
| Field | Value |
|---|---|
| Server / Domain | sip.volai.cz |
| Username | from the response above (username) |
| Password | from the response above (password) |
| Transport | UDP |
| Port | 5060 (default SIP port - leave blank if the field offers an empty option) |
Client also asks for a "realm"?
Most clients pick up the realm from the server's response on their own and never show you a separate field for it. If your client still shows it as a fillable field - sometimes labeled "domain" or "realm" - enter sip.odorik.cz there, even though the Server / Domain field above shows something else. It's set by the phone network, not volai, so it can differ - it's not a bug or a mismatch to fix, registration just won't verify without the correct value.
After saving, the softphone should register (an "online"/"registered" status icon) and calls to the number will start ringing directly in it.
3Outbound calls from a SIP client
Once the softphone is registered, it can dial out through this line on its own - you don't need to call any of our APIs for that, you dial directly from the client like from an ordinary phone line.
Minutes come out of your volai credit
Outbound calls placed through a SIP client are billed at the same rate as an outbound call through the API - 0.92 CZK/min (~EUR 0.04) and deducted from your credit - not from some separate SIP account. Rates exclude VAT, like the whole price list.
The connection runs directly between your client and the server sip.volai.cz, not through our API, so minutes can't be blocked at the exact moment of dialing. Automatic billing of calls dialed directly from a SIP client is still being finished - until it's done, such a call may not show up on your credit right away, but we'll bill it retroactively. When your credit drops to zero, new billable API/MCP actions start getting rejected (a call needs a balance of at least 5 CZK). If the debt exceeds 50 CZK, you'll get a warning email - and if it isn't settled within a week, the number is released back into stock (which permanently ends SIP access). So keep an eye on your credit ahead of time, not once it's already at zero.
Have a voice AI agent, not a softphone?
This step assumes a client that can register with the line (SIP REGISTER) - a softphone, a PBX. A platform that just sends an authenticated call once without a persistent registration (typically voice AI agents like ElevenLabs) needs a different approach - see Bring your own agent.
4Limits
- One registration per line - one set of credentials supports one registered device at a time. If you sign in with a second client, the first one will most likely lose its registration - don't rely on multiple devices on one line at once.
- Outbound from your own agent yes, full trunk peering no - the line's credentials let YOUR client register with US as a phone. Outbound calls from your own voice agent (ElevenLabs or another platform) DO work today though - through the relay, see Bring your own agent. You still can't connect your PBX as a peer carrier with its own routing for INBOUND calls from outside though - use
forwardrouting to your PBX's number for that. - Voice only - no video or other SIP extensions.