We have used
http://www.voiptalk.org/ for our IAX termination for many years and have never had issues. They do also offer SIP termination as well and can't imagine there would be much difference between the two protocols regarding DTMF.
If IAX is an option for you I would recommend switching to that for trunking.
Note: Am not affiliated with VoIPTalk at all just a user. I'm sure there are many other good providers out there.