For us the unit is running at a Fritzbox 7270, the application went without problems ...
If a call purely just on the green handset Press and already you take the call, the volume can be adjusted on the clip!
Now comes the clou ... should another DEC may be connected device to make calls with these "out" ...
and by "Consult" Connect the clip .... So on Mobieltelefon the number ring can then select the Mobieltelefon the key "R" press "** ???" choose mostly the extension has a number 600-630.
However, these settings can comb on the Fritzbox look ... then rings the clip press "green phone" and hang up on Mobieltelefon ... Done !!!
Short and sweet ..... CLEAR BUY !!!