A communications device such as a mobile telephone (1) responds to the input of a dialed number for a call destination (2) by generating and outputting a request message transmitted to a control centre (7) via a telecommunications system (3). The device receives a response message from the control centre containing routing data and credit data representing remaining credit within an account limit of the subscriber. An outgoing communications session is then initiated using the routing data which may for example enable a dialed telephone number to be modified to achieve optimum routing to the call destination via one or more preferred networks. The device also includes a credit control application (83) for limiting the duration of the communication in accordance with the credit data. Mobile telephones may thereby be used with a prepayment subscription account even during roaming operation in which the mobile telephone registers with a network other than the subscriber's home network.