No audio on incoming calls

I have both a Unidata and Cisco phone for BYOD. About 50% of the time, when I receive a call I answer but there is no audio. There is no audio on the other end either. I can make outgoing calls just fine on both devices and the incoming calls work fine the other 50% of the time. Does anybody have any suggestions? I can sit by a landline and call my phone 5 times after this happens to try and diagnose the issue but everthing works then someone else calls again an hour later and I have the issue. Thanks for any assistance.

Sounds like a nat issue on the RTP ports. What router/firewall do you have?

I have it on a DMZ of a Cisco E2000 router

If it's on the DMZ it's not what I thought. I don't know what the exact model phones you're using so I can't look up the manual for you to see what can be configured. If it were me I'd check the codec ordering next on your phone. IIRC on incoming calls your UCA makes the media offers, so if you had say 723 or gsm first Voipo might accept 723 but the media termination doesn't so you get no audio. I think cisco phone, at least on SCCP models but I'm sure sip as weel, will show you current codec on the rtp stats so if it's not 711 I'm willing to bet that's issue.

To sum up my early morning ramblings I'd check the order of codecs on the phone and make sure 711/PCM(a)(u) is first and if possible the only codec