A mobile device operates in a WLAN for VoIP communications, which includes registering with registrar server functionality of a SIP proxy server with a SIP identity which is further communicated to a location server. In response to an incoming voice call from a caller via a telephone network, the mobile device receives an INVITE request which is issued from a gateway and forwarded from the SIP proxy server. The mobile device sends a response to the SIP proxy server which is communicated to the caller via the gateway. When switching from the WLAN to the cellular network, the mobile device sends via the WLAN to the location server a message for forwarding voice calls directed to the SIP identity to the telephone number. The mobile device also sends via the cellular network a message for canceling the forwarding of voice calls directed to telephone number to the SIP identity.