Here's the NCT_IaxClientWrapper Library
This is a patch for build 566 of the IaxClient Library that adds a new audio type during initialization. The new type tells the iaxclient library to leave your port mixer settings alone rather than automatically resetting to the Microphone.
You must apply this diff and recompile (or use my dll) if you want to initalize using: AUDIO_INTERNAL_PA_PRESETMIXER
I'm not sure how this works with respect to the license so I may have to take it down. For now, here's a copy of a precompiled IaxClient.dll libary. Its built 566 with my own patch added.