A packet capture is the quickest way to get to the root of the issue. Because you are doing NSE-based passthrough your troubleshooting options are more limited as compared to fax relay where the helpful command "debug fax relay t30 all-level-1" is available. You confirmed earlier in the thread that you have the ATA and voice gateway configured correctly. Most of the time, problems such as yours are config issues. Can you post your configs so that we can double check your setup?
There are debugs on the ATA using prserv that will allow you to confirm the NSE switchover. You can also use the command "debug voip hpi" along with "no debug voip hpi stats" on the voice gateway (warning - this is an intensive debug so make sure you follow the appropriate debugging procedures before enabling this). The HPI debug can be difficult to decode but it shows IOS programming the DSP for passthrough as well as the NSE switchover messages. Additionally, you can repeat the commands "show call active voice brief" and "show voice call " as the passthrough call is occurring to check the DSP stats for issues. Unfortunately, there is not much else that you are going to be able to get from these devices for a passthrough call.
The packet capture is your best piece of troubleshooting information as it shows the NSE switchover messages, confirms a successful transition to G.711, shows network impairments that could be harming the call, and additionally you can extract the G.711 audio from the RTP stream and then figure out from the fax tones if there is a problem in the T.30 negotiation.
I understand your reluctance in posting packet captures and IP addresses on a forum such as this. You may feel more comfortable working directly with Cisco TAC on this type of issue. I hope the troubleshooting information above helps.
Regards,
David
