Hello mgs, and to everyone.
Let's suppose here, you're owner of swisscom's "Internet-Box Standard" and "Fritz!Fon C5" of AVM.
And respectively, please find instructions for a). Connecting a DETC telephone to the Internet-Box, and b). Registering FRITZ!Fon C5, get them ready aside.
1 ) Referring to the "Registering FRITZ!Fon C5", follow the step: 2 Using the menu to register a telephone - Registering FRITZ!Fon with a base station from another manufacturer. till the point 1.: Turn the telephone on and press the right select key "Register".
2 ) Wait until the message "Press and hold down DECT button on the base station!" appears on your C5. When it does,,,
3 ) Please press the " + " button of "InternetBox", as shown below in the picture. This will orders "pairing" operation.

3 ) Enter the PIN of the "Internet-Box" ( 0000 by default ) and confirm it with "OK" on C5.
- Process ends -
Hoping, this helps you 😉 with best regards,