Hi
Best of troubleshooting one way audio problem's is to determine the physical and logical path through which your media is going to flow.
Since by using MTP you have 2 way audio, there is a possibility that signalling should be fine with and without MTP.
Assuming you are facing problems with normal one to one call without any hold/transfer, I normally use following approach and set of questions to troubleshoot such issues :
1. Without MTP, who is unable to hear : your internal ip phone user or external PSTN/ITSP user : - If it's internal user : this tells me that your ITSP is having problems routing the RTP traffic into your network or your border router is unable to route the traffic into your internal network. IF external user, then most likely you are unable to route the RTP to your ITSP/PSTN.
2. When in call you can press the question mark button on your ip phone twice to get the call stats and this will tell if you are sendind and recieving packets from and to the ip phone respectively
3. Using MTP changes your routing path as the RTP traffic has one more hop through which it needs to go. Determine the difference between the two paths
4. Take packet captures on devices that you feel RTP should be flowing through and check if you can hear the incoming and outgoing audio those devices.
5. Press hold and trasfer when call is establish and check if this solves your problem.
This way you can greatly minimize the problem and device.
Regards
Aditya
