Have you tried removing/replacing/charging the batteries in the handsets? If fitting a brand new battery you should press 9 # 9 # 9 * 6 5 5 OK in order to let the phone know it is charging a new battery.
Can I also check how you are trying to connect? I haven’t used my ISDN for a while, but I think that it is only ‘open for registration’ for a minute or so after connecting the power. (This is unlike the PSTN bases which have a physical button to press to open registration)
EDIT: I checked the manual – there is also a light on the rear of the ISDN that flashes whilst open for registration – is this happening?
A further thought that might enable you to test the handsets: If your internet modem/router has built in DECT (many do), then you may be able to connect your Beocom 6000s straight to it cordlessly without using the B&O ISDN base. I have used Beocom 6000s like this in the past (with a German router) – there will be reduced functionality but calls in/out should work fine. At least this would check that the handsets work.
