Hi Ampers, first, which Gigaset handset are you using?
When you say the phone wire comes into the Gigaset box, does this wire connect to your BT wall jack?
The usual topology to for an analogue phone with the OBi and BT line would be to connect the analogue phone to the OBi and then connect the BT line to the OBiLINE. Any further details you can give about your setup will certainly help to solve the problem.
- Mark.