Sometimes we unfortunately still have legacy requirements to support ISDN that still need to upgrade to SIP Trunking. DTMF and even fax tones can be a pain to troubleshoot.
This guide from the Cisco Support Forums is brilliant, so I’ll just send you straight to it:
I will however make one specific note from the guide that is of key importance:
If there are still doubts on the DTMF being received by the gateway or not, move ahead with “PCM Capture on ISDN” however keep in mind that there is no 3rd party tool available to decode the PCM captures. You need to involve Cisco TAC to decode the PCM captures.
With this in mind, if the debugs show you nothing, you have no other option than to go to TAC to analyze PCM. Often this is the case for advanced Faxing tshooting over ISDN as well.
Hope this helps, it helped me a lot!