Because the FRITZ! Fon MT-F has only one socket for a 3.5 mm jack, can be directly connected as good as a normal headset. But with this adapter, it is possible and I was able to connect the Sennheiser PC3 CHAT headset that has two 3.5 jack. Safe as well as other headsets in this way to the FRITZ! Fon MT-F can be connected. Important are the two jack on the headset. With USB port, it of course does not work.
Important perhaps for purchase here at Amazon. I have ordered from the supplier "adaptare" as here recommended by some Amazon reviewers.