Here is a thread I created a few months ago with the same problem.
IAX trunk suddenly failing in one direction with CAUSE : No authority found CAUSE CODE : 50
Used to work before.
One system is Asterisk 13, the one the call is failing to is Asterisk 11.
What are your Asterisk versions?
Also try type=friend instead of peer.
Can’t offer a solution, just looking for things we have in common in our setups.