our IAX2 support is limited.
We are using our own fork of the iaxclient library from http://iaxclient.wiki.sourceforge.net/

This is the client lib used in most iax soft phones and is not suitable as-is for a server.  To avoid causing commotion for the other software that uses this lib we just forked it and added our own mutexes and a few other things.  (this is not a hostile fork as I have commit access on the real one. It's just because there are several experimental changes.)

Even with our changes, the original lib has no trunking support and still barely qualifies to be used in a server so it's not worth it to add the support.

The real solution is to write an entire IAX2 stack from scratch that actually can scale for use on a server.  This is within my capability but not prudent because the IAX2 protocol is, in reality, an Asterisk Specific proprietary protocol and it wouldn't be fair for us to have a nicer IAX2 implementation than them.  Not to mention, there is nearly no demand for this protocol among our user base. 

There was an openiax http://www.openiax.org that was underway.  Maybe someday if that gets off the ground we can look at supporting it.  But all signs point to the fact that we would need major motivation to expand our current iax2 support beyond what it currently is.