Mark,
Let me start by answering another members point and that is Beotalk 1200 is an analogue line only product and as such cannot be used on lines like ISDN.
With regards to the cable issue, don't forget that in the UK pin 3 on the BT type phone plug is "Ring" and without pin 3 connected the Beotalk 1200 will not answer any calls. Pins 2 & 5 are the line and pin 3 is created in the Master Socket via a capactor.
So, if you have your Beotalk 1200 plugged into a "Slave" socket without pin 3 connected, this could be your problem.
With regards to the BT (6 pin) to RJ11 (4 pin) cable, pins 1& 6 on the BT type plug are not used, but the rest of the pins are in order, in other words pin 2 of the BT plug goes to pin 1 of the RJ11 plug. Actually on mind I use a RJ11 to RJ11 cable (4 wire) with an RJ11 to BT adapter.
I suggest that you use a standard and cheap 4 wire RJ11 to RJ11 cable with a RJ11 to BT adapter such as the one you see HERE
If that still does not work then plug your BeoTalk 1200 into your Master Socket