I was reading through the instructions for the Vigor 2930 and if I'm right it says that the ISDN ports (with the orange light flashing) can be used to connect an ISDN telephone to the router. I've tried plugging a phone in and it doesn't seem to work. When I pick up the handset it's display reads 'Please dial' for a second or so, then 'Call cleared', I don't even get a dial-tone. I've tried the phone in an ISDN PABX and it works there. Is there something obvious I might be missing?