Thank you Leonid, that did the trick. After analyzing the MIME packet using your code from the other thread I can read the DTMF input. After adding code to acknowledge the SIP INFO packet I am getting the next one and am now receiving DTMF tones via SIP INFO tones correctly.
For everyone else finding this thread it might be worth noting, that you have to enable MSGINFO and MIME access in your virtboard before starting the global call lib:
virtBoard[0].sip_msginfo_mask |= IP_SIP_MSGINFO_ENABLE;
virtBoard[0].sip_msginfo_mask |= IP_SIP_MIME_ENABLE;
I still have an issue where DTMF tones leak into the callers microphone and trigger a (duplicate) TDX_CST event, so I'll probably block those when I have received GCEV_CALLINFO with dial tones, assuming that once the caller sends DTMF via SIP INFO he won't be sending in-band DTMF as well.