I didn't touch dtmf settings since I realised that the problem was with my phone. Sure, it's strange, but it seems to be working on other phones just well. My phone is Siemens Gigaset SL100, connected through Asterisk (via cisco ATA), but I also tried with direct connection to pots... I haven't had time to look into it more deeply, I just left it the way it is.
I'll try with