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A trunk is a carrier connection: inbound, outbound, or both. Trunks
No credential is ever rendered. A trunk’s password is stored encrypted and never returned by the API, so this screen shows what a trunk is and never what it authenticates with.

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Your own carrier

You may bring your own trunk. Calls over it cost the platform nothing, so:
  • It is preferred over the operator’s when both could carry the call.
  • It works even when your balance is empty.
A tenant’s own trunk is never selectable by another tenant. That is enforced in carrier selection rather than hidden in the interface — otherwise it would be unmetered calling on somebody else’s account.

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What a carrier may need from you

Five settings exist because carriers differ, and all five are on the trunk:
  • Dial prefix — digits this carrier wants in front of the number, added on the way out. Not the same as your organisation’s outside-line prefix, which is what staff dial to reach a line and is stripped on the way in.
  • Accept national-format numbers — for a carrier that hands over 09216217231 rather than +919216217231. List the Countries its numbers belong to: AU, or AU, IN for a carrier that delivers both. A national number is tried in each country, in order. Only a number you own can match, so two readings of the same digits can’t both ring. If you own both, the first-listed country wins.
  • Outbound channel limit — reserves part of the trunk for calls coming in, so a burst of outgoing calls cannot fill it.
  • Number format — international digits (the default), international with a +, or national (0412345678) for a carrier that takes its own country’s numbers that way. With several countries, only the first (the carrier’s home country) is sent in national form. Every other country’s numbers go in full international form, so the carrier never has to guess which country a 0 number belongs to.
  • Default caller ID — the number shown to this carrier when neither the extension nor your organisation has one; usually the number the carrier account is registered as. Without any, a call presents the extension number and is usually refused.
Your own trunk carries only your numbers inbound: a call to another organisation’s number over it is refused. If a carrier sells several service levels (tech prefixes), list them on the trunk’s page and choose between them by destination with outbound rules on Outbound routing — +91 at one level, everything else at another. Its page also shows what it is doing right now: whether it is registered, how many channels are up, who is allowed to call in on it, and what it has carried.

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Failover

Trunk groups try carriers in order and fall through on refusal. A call that no carrier will take ends with the reason on the call record rather than as silence: the call’s page shows Far end, the carrier’s own answer (403, 503), and the caller hears that the call cannot be completed.

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WhatsApp calls (Meta)

Meta delivers WhatsApp calls over SIP like any other carrier:
  • The trunk: Transport SIP, authentication by IP address, with Meta’s SIP source addresses as allowed addresses. Meta connects over TLS, normally to port 5061.
  • Codecs: WhatsApp audio is Opus, so tick Opus and keep it first in the order; G.711 behind it does no harm. Do not leave the codecs unticked: an empty selection means G.711 only, which a WhatsApp call cannot use.
  • Encryption: in Meta’s calling settings for the number, set the SIP server’s SRTP key exchange (srtp_key_exchange_protocol) to SDES.
With Meta’s default, DTLS, the call connects on the caller’s phone and they hear nothing. The switch sends its answer and then waits for Meta’s DTLS handshake, which does not complete. FireTone gives up after 15 seconds and logs call had no audio with leg=dtls_not_completed and the fix. The call’s trace shows the same under media.
A call that was answered but never got its audio up for another reason is logged as answer_not_completed, with what the far end offered: its transport, encryption and codecs. Compare those with the trunk’s codec list.