First make a FISCON factory reset as it is possible that already 5 phones are paired which is the limit. Some versions of Android use a deviate pairing method.This can cause the problem of a failing connection.
The solution is the free tool Bluetooth File Transfer (Android) for a better Bluetooth connection. Establish the pairing once with this app and from then, everything should work as normal.
In addition an update for the FISCON firmware can fix the problem.
To activate this feature check the bluetooth connection profile for FISCON on the phone.
The handsfree has to be connected with the Plug&Play harness, depending on the car, either behind the radio or CD-changer.
It is not required to remove them completely from the car.