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Blog Carriers and SIP

Bring your own SIP trunk: how Vocivo connects to your carrier

· 6 min read · Vocivo team

Most hosted phone systems sell you numbers and minutes. Vocivo works the other way round. You keep your contract with a local carrier, you keep your numbers, and Vocivo becomes the phone system on that line. The carrier deals only with you.

What a SIP trunk is

A SIP trunk is a phone line delivered over the internet. Your carrier sends incoming calls to an address you give it, and accepts outgoing calls from that address. SIP carries the signalling: who is calling whom, and whether the call was answered. The voice itself travels alongside as RTP audio.

Vocivo owns no numbers, no carrier accounts and no carrier-facing IP addresses on your behalf. The trunk is yours, contracted in your company's name with a SIP trunk provider licensed in your country. Whichever provider you choose, in Saudi Arabia, the United States or anywhere else, Vocivo connects to it in the same way.

The two ways carriers connect a trunk

IP authentication

The carrier trusts calls that come from a fixed public address, and sends your incoming calls to that same address. There is no username or password, and the trunk never registers. Many local carriers work this way: your SIP trunk provider authorises an address on your account and signals to it on UDP 5060.

Because such a trunk never registers, Vocivo's trunk screen says so plainly instead of showing a red light forever.

Registration

Vocivo signs in to the carrier with a username and password, the way a desk phone signs in to a phone system, and the carrier sends calls to wherever that sign-in came from. Many SIP trunk providers support this. Vocivo keeps the carrier password encrypted at rest with AES-256-GCM and never shows it again after you save it.

One number, four spellings

The same Saudi landline can be written at least four ways:

  • +966130000000 E.164, the international standard
  • 966130000000 international, without the plus
  • 00966130000000 international, with the 00 exit code
  • 0130000000 national, as a local phone dials it

Carriers disagree about which one they want. Some refuse the + sign both in the number dialled and in the caller ID, and send callers to you as 00966…. So every Vocivo trunk has its own number format for the number dialled and for the caller ID, and a carrier country.

Your people dial like a local phone. Vocivo rewrites each number the way that trunk's carrier expects on the way out, and records incoming callers in E.164 so your call history can always call them back. Emergency numbers such as 911, 997 or 999 go out exactly as dialled.

Codecs

Carriers almost always speak G.711: PCMA (A-law, used across most of the world) and PCMU (µ-law, used in North America). Vocivo offers the carrier exactly the codecs set on the trunk, plus telephone-event for keypad tones, so an IVR at the other end hears your digits.

Caller ID

Caller ID is set per number and per person. A direct line can present itself while the main line presents the main number. Vocivo sends the caller ID in the From header and in P-Asserted-Identity, which is where most carriers look for it.

What to ask your carrier

Send your carrier these questions before you start. The answers fill in Vocivo's trunk screen.

  1. Is the trunk IP-authenticated or registration-based? For registration: the username, password and registrar domain.
  2. Your carrier's signalling address, port and transport. UDP 5060 is the most common.
  3. Which address of ours they must authorise. Vocivo gives you this address, or it is your own server's static IP if you self-host.
  4. How they write numbers: the number dialled, the caller ID, and what they send to us on incoming calls.
  5. Which codecs they accept, usually PCMA and PCMU.
  6. Your numbers, and which of them you may present as caller ID.
  7. How they route emergency calls, and whether each number needs a registered address.

After the carrier answers

Enter the details on the SIP trunks screen and deploy the gateway. Your numbers arrive in inventory unassigned, and each one gets its own destination: a person, a ring group, a queue, a menu or the AI receptionist. Then test each number separately. Record an outbound call, outbound audio, an inbound call to each number, inbound audio and keypad tones. A working first number tells you nothing about the fourth.