IAX2

IAX2 stands for Inter-Asterisk eXchange version 2. It's a voice protocol built specifically for Asterisk, the open-source phone engine that sits underneath VICIdial. Its main trick is carrying both the call signaling and the actual audio over a single network port, where sip splits those across separate streams that have to be coordinated. One protocol, one port, both jobs.

That single-port design makes IAX2 noticeably friendlier with firewalls and home or office routers. SIP audio often gets tangled up in network address translation, where a router has to guess which device an incoming packet really belongs to — the classic cause of one-way audio. IAX2 sidesteps most of that, which is why it became popular for linking two Asterisk servers together over the public internet without a lot of network plumbing.

In practice, though, most VICIdial deployments today reach the outside world over SIP. Almost every phone provider sells SIP trunks, and far fewer support IAX2 at all. So while you might still see IAX2 used to connect servers inside your own setup — say a dialer box talking to a separate media box — the trunk that actually carries calls out to your phone provider is almost always SIP. If you're shopping for a provider, don't expect IAX2 support to be on the menu.

If you do run IAX2 somewhere in your setup, keep in mind it doesn't make any of the usual capacity problems disappear. You'll still see dropped audio, choppy sound, or congestion when too many calls hit at once, because those limits come from your hardware and your provider, not the protocol. The protocol choice also doesn't change how many simultaneous calls your carrier will accept or how many your did numbers can handle at one time. Treat IAX2 as a connection style between systems, not a way around your real capacity limits.