I have spent a lot of time getting explanation for this case:
We have two PBXs (both AstX) and IAX2 connection with between them. The both servers are Xenons with plenty of RAM. The SIP trunk is connected to the 1st PBX that forwards most of the calls to the 2nd server (using IAX2) the calls are excellent quality (current load is low 1-20 simultanuos calls). PBX2 forwards back some of these incoming calls like 50%. It uses the same IAX2-connection to forward back calls to the 1st server. After half an hour (this has occurred several times) the calls start to get distorted and after a couple of minutes once distortion starts all the calls are totally messed.
I Spent time googling, but I found very little worth of reading and up to date matching our case… I know the tech detail level I provide is quite high but the one who has encountered this kind of behavior will defenitely recognize it.
Just some sanity testing
*iax2 packet loss in 20-30% gives the same quality in calls
*Iperf3 runs rate of 100MB without any packet loss (udp)
*Wireshark doesnt show any abnormal traffic in the network.