Random IVR Failure Troubleshooting

I have received complaints from a few of our customers that our IVR never works for them. It always appears to be the same customers and only a select few.

Because of it not effecting everyone, I want to believe that it is not an issue on my end - but I would like to be able to prove it.

We have a SIP trunk from our T-1 provider. It is using dtmfmode=rfc2833 - which I believe indicates that the DTMF detection is done on the providers side and the tones are transmitted digitally, correct?

How can I log the DTMF codes that I receive?

Any other ideas on how to troubleshoot this?