Dreamland,
From the CAA:
'Sleeping' Receivers
A series of incidents has been reported where ATC was unable to contact an aircraft that had previously established two-way communication with the ground controller. In almost every case, satisfactory reception was only restored after a transmission from the affected aircraft. ... The CAA is aware of more than 250 incidents of missed calls since 1999. CAA Air Traffic specialists led a team with representatives from NATS, EUROCONTROL, Thales and British Airways to investigate this issue and recommend actions to CAA to address 'sleeping receivers' causing prolonged loss of communication.
The investigation revealed that on a small but critical percentage of occasions, the aircraft communications transceiver failed to return from the transmitting to the receiving state. To mitigate this problem, one transceiver manufacturer has devised and published a non-mandatory service bulletin. The recent incorporation of this service bulletin into the ATC transceivers carried by a major UK airline has proved to be completely successful, but the CAA believes that this problem is very likely to be replicated in other transceivers...