In short, a SIP trunk is a virtual phone line, which uses the SIP signalling standard to pass calls over the internet. The easiest way to view a trunk is that it is an internet phone line, that can handle multiple calls at the same time.
Not all trunks use SIP, although most do. In the past IAX (Inter-Asterisk exchange) trunks were popular, but as the name implies, were limited mainly to Asterisk servers. This format was mainly used for communication between asterisk servers, hence the name.
It is still in use today, on version 2 (IAX2) but not as much, as the format is not very flexible. It is popular as it uses a single UDP data stream for both signalling and audio traffic, which made its use through firewalls much easier to configure and manage.
As we do not use Asterisk hardware within our network, we do not support IAX trunking.