That just makes no sense. That's not how it works. OBiBT is not assigned to a phone jack, neither on your OBi nor on your cordless phone.
You can pair your cell phone to your OBiBT, via Bluetooth, to use your cell phone as a service provider, just as if it was a VoIP service provider, to place or receive calls over your mobile phone carrier's network.
If you want a cordless phone that interfaces with a cell phone over Bluetooth, then buy a cordless phone with built-in Bluetooth capability.