I've played little bit with DTMF decoding and sending little bit and find out what SIM900 module which I use - have software level DTMF decoder.
So, all you need is just send right AT+ command to the module, and it will start to decode DTMF signals (in perfect world).
As result I don't need dedicated decoder at all.
On the other side it is not so reliable as I wanted.
Another thing I learned - this particular GSM module didn't work without SIM card at all. Which is really sad.
Firmware for first aid kit is almost ready and waiting for to be pushed to GIT repository.