@Wash, they support dtmf, as their voice mail system runs IVR. Have you tried a different codec, or pls check the log of the  gateway. It is strongly possible that they dont support dtmf inbound except for their own tools. I had a run in with Kencell a few years ago when designing an inbound data sessions system. They were blocking both CSD and GPRS inbound on their network and it never got resolved. I had to move to safcom for this. And also discovered that most ATMs were using safcom data CSD for their sessions.