I agree - changing the codec makes no difference at all. But there are some holes in your story. You're saying that without STUN, you can't make a call at all - but, you then say you're on the same network.
Those two things don't make sense. If you need STUN, then there is a firewall in the way, and NAT.
You've also changed the RTP ports that "is in the dynamic port range that other systems occasionally use". That ... worries me too. What other systems? Why does it matter? Is it going through NAT or a Firewall after all? You said it wasn't.
You appear to be doing an amazingly good job of overcomplicating things. This always leads to trouble, as you are now encountering.