BT Revelation Owner's Handbook Manual page 45

Check that all ISDN devices work correctly. If
they do not, report the problem to your
network provider.
Line busy (in use by ISDN device); no LED
indication
Incoming VOICE calls answereed by an ISDN
device will not light the line key (LED) on the
Systemphone. Incoming data calls answered
by an ISDN device will light the line key on
the Systemphone.
43