The best debug is 'debug voip rtp session named'.
You'll see something like <<<<< 101 >>>>> Ev: 2
Meaning: RTP payload type 101 with an event of 2. 101 is the normal 2833 PT, and Event 2 means digit 2.
Be aware that for redundancy most systems send multiple copies of the packet, and sometimes include Begin and End to the packets. So, you'll see quite a few of the digits come in. 'debug voip ccapi inout' should help you determine what are being interpreted to digits to the call API level.
