According to one aspect, a communication network system comprising at least one first network and at least one second network providing application services over the first network is described. The system comprises first network means (GGSN) for receiving a message associated with an IP address allocated by the first network from the second network, for detecting that there is no active communication channel for this IP address which is allocated to a user equipment, and for sending an indication that the user equipment is not reachable through this IP address to the second network. The system further comprises second network means (P-CSCF/PCF) for receiving this indication.