What cables are you using to connect the pstn unit to the telephone connection.
BT in its wisdom have chosen their own standard on socket wiring, which is very different from the rest of the world! Bless BT 
The rj11 connector used by BT have the 2 phone wires connected to outside pins, pin 1 and 4 and the earth and bell signal to the two inner pins, 2 and 3 .
This is a source of issues when using UK phones abroad or visa versa .
Not sure what the B&O supplied cables in the UK are wired. I have US B&O set in the UK so knew what to do to make them work.
You might simply get it to work by avoiding UK type cable ( brought with you from UK) and use locally sourced cables.
Regards
Peter