Sorry for the delay in posting back.

On my adaptor, the middle two pins of the RJ14 plug (positions 3 & 4, red and green cable colours respectively) are cabled to the first BT socket (pins 5 & 2 resp), whereas positions 2 and 5 of the RJ14 (black and yellow colours resp) are cabled to the second BT socket (again, pins 5 & 2 resp). The adaptor also contains two capacitors, each cabled between pins 2 and 3 of the BT sockets.

This arrangement suggests that a phone lead with a RJ plug that has just two contacts should connect to the first of the FXS ports. A RJ plug with four contacts should also work if the phone is presented on the two inner contacts (for FXS port 1) or the two outer contacts (for FXS port 2) — but any other combination probably wouldn't work.

Regards

Thanks for that D Malone. The 2820vn seems to be wired in the standard RJ14 manner.

For anyone searching on this subject in the future compatible adapters seem to be commonly refered to as "Triplex Adapters" in the US and are cheaply available in the UK at places like Amazon.