I had this sort of distorted DTMFs on certain IVRs I called - on a different VoIP provider, and with a Linksys PAP2, and with in-band DTMF. They incorrectly diagnosed the issue, changed me to SIP signaling (out of band) - and that proved incompatible with ANI used by some of their carriers.

But on VoIPo, with the Grandstream, no issues so far, including services (IVRs) I call and a conference bridging company I frequent (800 number)