Thanks for the reply.
Contacted avm support and they informed me that the option is not available through GUI.
I am assuming it is available through other channels such as config file editor or Freetz...
Will try to find any info in config file...
The sim card in an android phone works fine and sends successfully dtmf codes.
I have a Gigaset connected directly to F!B via DECT. There is no any option regarding DTMF codes in
the menu of the phone.
DTMF cards also does not work via Fon app for F!B.
Will try to use Gigaset via its own base and Fon port to see if it makes any change.
I expected that F!F C6 would have more options and better integration than Gigaset to F!B via DECT.
If I can remember correctly, Panasonic is even worse with integration to F!B via DECT.
It would be better if manufacturers followed the standards without making it proprietary.
It reminds me on the time when people carried small buzzers in the pocket for that purpose. I thought the time is long gone.