TRUNKS calling out in Asterisk

Trunk connections from your in house Asterisk box to a service provider seems to be black art. The instructions available on the 'NET seem so simple, but why doesn't it work for us?

What is a trunk? No, it isn't something used for luggage on boat cruises.

A trunk is a physical path or link in a communications system that is designed to handle many transmissions simultaneously and that interconnects major switching centers or nodes. Depending on the system, a trunk may carry transmissions in analog or digital form. Transmission content may include voice (as in the conventional telephone system), text, computer programs, images, video or control signals.

Setting up a 'trunk' line between ourselves and the outside world is complicated by the fact that the outside world isn't using the same equipment as us, and don't necessarily speak our PBX Asterisk, the way we want them to.

Yeah, it works, but why does it no work when it doesn't, and what about these strange misbehaviours?

When you're pushed to get things working it doesn't help that the crisis is on production equipment and there is little leeway (opportunity) to 'fiddle' with the working environment to make sure your expectations are correct.

  • What does this knob/option really do?
  • What happens if it is off or on?

Fortunately, our office commissioned another SIP Trunk and made some onerous requirements for its use. This forced our hand to spend some more quality time with our firewall settings, asterisk settings, watch tcpdumps (both using OpenBSD's tcpdump and wireshark) just to get the thing working correctly.

Now, we have have some clearer internal documentation for configuring Trunk with a SIP Provider, and updated our Maintenance to include a short brief on using tcpdump(8) with diagnostics/analysis.

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