Om has a very important post about IAX vs. SIP but he should have asked me.
A few years back I served as the USA consultant for an Israeli softswitch company, Comgates which has since been acquired by another telephony company. While Comgates didn’t ever reach their vision of founder Jacob Tirosh, one point that Tirosh always made was that a softswitch had to be protocol agnostic and he created the concept of MultiProtocol On The Fly.
The idea of Multiprotocol On the Fly was simple. The ability to switch protocols as needed, and as conditions of the network demanded. For example, if the call originated as h.323 but needed to terminate on a SIP network, the Comgates switch would handle the interworking function. If the best way to route the call was over a SIP based network, but the call was h.323 on both ends the switch would do that, all without adding any latency or cause rebuffering. While this is an over simplification, the idea of MultiProtocol on the Fly clearly was ahead of its time.