A mobile station is operated within at least one radio network having a plurality of base stations. Only some of the plurality of base stations support a Voice over Internet Protocol, VoIP, service. The mobile station is registered with a first base station of the plurality of base stations. The mobile station may be registered with a second base station. The mobile station determines that one of the first base station and the second base station has functionality to support VoIP and the other does not. A Routing Area Update Request message or Tracking Area Update Request message is sent from the mobile station to the second base station in response to determining support for VoIP.