Tito,

I wish I was in a position to answer you correctly on your question but I am not a telecomms engineer so I know I can't. Looking at the DTMF parameters configuration, I can only see DTMF types - Alphanum and Signal. Not sure what is what there:-) However, on the destination, I can see the option to enable/disable "out of band" DTMF. My argument is only based on the actual situation I am in - mostly from the actual tests I carried out. I'll have to read volumes to be able to answer you.
My GSM gateway is a 2N Voice Blue Enterprise, Model 505204E. It has 4 MC39i Siemens GSM phones inside it, thus it takes 4 SIM cards. Maybe you can check it out at www.2n.cz, but what my concern was about is the fact that from my testing:

From Zain --> Zain, DTMF not working
From Zain --> Safaricom, DTMF working
From Zain --> Telkom, DTMF working
From Safaricom --> Safaricom, DTMF working
From Safaricom --> Zain, DTMF not working

@Aki, I have tried changing the codecs between G.711 ALAW 64k and G.711 ULAW 64k (for those are the ones supported between the GSM and the PBX) but that did not change the equation. At least I don't have a codecs mismatch:)

@Muchai, have you managed to ping those folks at Zain? I'd be most grateful since this project is taking all my time.