I can tell you how I solved the problem I was having.  Don't know if this is preferred, or if there is a better way, but I'm now able to detect dtmf from all sources (pstn, cell, sip).

Apparently, mod spandsp has better dtmf detection than the dtmf detection in the freeswitch core.  On the wiki page for spandsp (http://wiki.freeswitch.org/wiki/Mod_spandsp), there is a section on dtmf.  I added the spandsp_start_dtmf parameter before calling the ivr, and that fixed my issue.

Hope this helps,

-- 
WBR,
Viacheslav Dubrovskyi