NAT: IAX beats SIP 6-0-6-0-6-0

In my opinion the biggest advantage of IAX is the ability to adapt to any network topology. As Mark Spencer states:

IAX's unified signaling and audio paths permit it to transparently navigate NAT's and provide a firewall administrator only a SINGLE port to have to open to permit its use. It requires an IAX client to know absolutely nothing about the network that it is on to operate. More clearly stated, there is NEVER a situation that can be created with a firewall in which IAX cannot complete a call nor be unable to pass audio (except of course if there was insufficient bandwidth).

And regarding bandwidth:

IAX is more efficient on the wire than RTP for any number of calls, any codec. The benefit is anywhere from 2.4k for a single call to approximately tripling the number of calls per megabit for G.729 when measured to the MAC level when running trunk mode.

More info here: http://www.voip-info.org/wiki/view/IAX+versus+SIP