Intermittent one-way audio issues Resolved!

So if something is 99% of the time related to a certain issue it is probably the cause right?  Well, being stubborn and convinced it wasn't that 99% I found out today it was.  I did a PCM capture of the phone call exhibiting one way audio and you can hear both parties talking but yet only one way audio.  I did a debug mgcp packets and saw the channel in use and everything looked good there.  ISDN Q931 was clean and there were no transcoders or MTPs involved so what is the issue you might ask?

Well, 99% of one-way audio issues are network related.  I didn't adhere to that rule since I was so sure it was something else.  I figured since the issue followed the phone it couldn't possibly be the network!  I didn't stop to consider that when pushing it out the PRI, the network in use is still that same gateway.  It turns out the customer had two default ip route statements, one pushing to an IP address and another pushing it over the wire via interface.  We pulled the 0.0.0.0 0.0.0.0 fa x/x route out of there and everything started to magically work.  Now the strange part is that it really shouldn't have mattered unless there is something I am missing?  I'm not a CCNP R/S so I might be talking out by bum here.

Lesson learned: If its 99% possible that something is the cause, go check the damn 99%!