[VoiceOps] Calls to China degrade after 5 minutes

I am experiencing INTL calls to China degrade after 5 minutes. First few minutes is crystal clear, then after that, it gets worse, and then unusable. I tried changing upstream transit providers, and SIP trunk carriers, and it didn't help. I am going to try a few more things, but anyone have any experience here? Is this a known issue? This has been an ongoing issue, not something that just popped up. Jim Gurol California Telecom 213-270-1349