True. I am ignorant on this area as I already mentioned that I am not a telecoms engineer:-)
I have been talking to Vincent Mayabi of Zain. I guess you work with him. He's been able to test the system and did not succeed -- same way I did not. Initially he wanted to run a test with older type SIM card - 32KB or something. I have a SIM card which I think might meet that criteria - the old "Yes!" SIM cards. This did not work. Now what he's told me is that:

(1) DTMF would only work for me if I was connected to Zain via an E1 (like Call Centres do).
     I am not sure I can go that direction.
(2) I need an equipment that supports out-of-band DTMF. Now, that is another long shot! I have sent a query to the manufacturer to tell me whether the equipment does handle both in-band and out-of-band DTMF.

Again, I tried to argue (as a layman) why it is that this GSM gateway would work well in the scenario where I have a Safaricom line in it. In this scenario, calling it from Safaricom, from Zain and from Telkom works okay. BUT when I have a Zain line in it then calling from Zain or Safaricom does not work. He told me that Safaricom must be supporting in-band DTMF while Zain does not. Why? Because that's an internal decision within Zain.

If you can help unravel the mystery, I'd be so happy as honestly, implementing this service with a Safaricom line means I have to get a couple of guys who are going to rely on this system to discard their Zain lines and start using Safaricom. I'm talking about 20 people,which I think is significant and that gives me headache elsewhere.

Looking forward to your advise as I find this quite confusing.