Voice & Calls
Receive and place phone (PSTN) calls on your numbers, route inbound calls to a phone, a SIP endpoint or an AI voice agent, and read every call from one history.
Receive and place phone (PSTN) calls on the numbers you buy. Calling is on as soon as a number is provisioned. Inbound calls route to forwardTo on the number (a phone, a SIP endpoint or an AI voice agent); outbound calls dial from the number with POST /v1/voice/calls or from a browser softphone. Every call, inbound and outbound, is listed by GET /v1/calls with optional recording and transcription.
This is standalone phone calling, available in every country Zernio sells numbers in. WhatsApp Business Calling is a different feature: voice calls placed inside a WhatsApp chat, which Meta blocks for outbound in the US, Canada, Egypt, Vietnam and Nigeria. Both channels appear in the same call history.
Quick reference
| Property | Value |
|---|---|
| Directions | Inbound and outbound |
| Inbound destinations | Phone (tel:), SIP endpoint or hosted agent (sip:), or your own media server (wss:) |
| Outbound | POST /v1/voice/calls, or the in-browser softphone (WebRTC) |
| AI agents | SIP (Vapi, Retell, ElevenLabs) or a WebSocket media server of your own |
| Extras | Voicemail, business hours, IVR menu, caller blocklist |
| Recording | Off by default, +$0.004 per minute |
| Transcription | Off by default, +$0.03 per minute |
| History | One feed across every number and both channels |
Before you start
Voice requires usage-based billing with a payment method on file. Calls are metered per minute at the rate for the route; the call rate table lists every destination.
In this section
Setup
Set where inbound calls go, plus voicemail, business hours, an IVR menu and a blocklist
AI voice agents
Bridge calls to a Vapi agent over sip:, or your own media server over wss://
Integrations
Put an ElevenLabs or Retell agent on a Zernio number over SIP
Outbound calls
Place, transfer and end calls with the API
Browser calling
Dial from a web page with the WebRTC softphone
Call history and recordings
List calls across every number and fetch recordings
Related
- Enable voice on a number: every routing field.
- Place an outbound call: dial from your number.
- Mint a browser softphone session: the WebRTC handshake.
- List all calls: every call, both channels.
- Call webhooks:
call.received,call.endedandcall.failed.
Branded Calling
Show your business name, logo and reason for calling on the callee's phone: register the business, create a caller identity, pass vetting, then attach your US numbers.
Setup
Set where a number's inbound calls go and turn on voicemail, business hours, an IVR menu, a caller blocklist, recording and transcription.