Measures for use in operating a network node in a telecommunications network. At a network node located in a second part of the network, signaling messages transmitted between a server and end devices in an at least a first group are monitored. Each monitored signaling message includes a telephone dialing number for a given end device in the at least first group and an extension number for the given end device. On the basis of the monitoring, mapping data is stored which includes mappings between telephone dialing numbers and extension numbers for end devices in the at least first group. In response to detecting a loss of connectivity between a first part of the telecommunications network and the second part of the telecommunications network, extension number dialing request messages received from end devices in the first group are processed according to a detected loss of connectivity operating mode.
