A mobile first terminal (1) has a call in progress with a second terminal (2) under a first communications service via a base station (10) of the access network of a first subsystem. A condition for transferring the call to a base station (20) of the radio access network of a second subsystem is detected in a radio network controller (11) of the first subsystem. A core network switch (12) which is linked to the radio network controller (11) of the first subsystem is informed of said detection of a call transfer condition. If the second subsystem cannot process the call under the first communications service, a service change is requested so that said call can continue under the second communications service.