The easy way... Go to Settings - Telephony - Internet telephony - Advanced settings
There is an option to choose * Automatic selection of voice codecs, * Always use fixed line quality (this is in fact G711) or * Always use compression (this is G723)
If your DSL bandwith allows the speed try use * Always fixed line quality ("Immer Festnetz qualitaet")
There is a more complicated way to edit the codecs, not using the web interface. For this use "the-construct" pseudo image, you can set any codec and order there in a pseudo kind of upgrade. Please look in the forum for more help on this, but I doubt if my fixed line solution does not solve your problem that codecs are your issue.
Have you tried this - pressing a second time on star after the first one? Some phones enable DTMF only after the first star, and start in pulse (rotary) mode.