I came across this a while back when deep diving MTP when preparing for the lab.  I’ve pretty much got it down now, but I keep coming back to it as a reference point.

http://www.cisco.com/c/en/us/support/docs/unified-communications/unified-communications-manager-callmanager/118708-technote-dtmf-00.html

I really struggled a lot with this at the CCNP level.  Not sure about MTPs conceptually?  Look no further!