A method for use by a mobile communication device in facilitating voice calls through a wireless local area network "WLAN" and a wireless wide area network "WWAN", the mobile communication device having a Session Initiation Protocol "SIP" identity for identification in the WLAN and a WWAN identification number for identification in the WWAN, the WLAN being adapted to provide Voice over IP (VoIP) communications for the mobile communication device and including a SIP proxy server, a location server, and a gateway for communication with a telephone network, the WLAN and the WWAN being connected to the Internet, the mobile communication device having a data communication capability in the WWAN, the method comprising the acts of: operating in the WLAN for communications, which includes: registering with registrar server functionality of the SIP proxy server using the SIP identity which has a format of an electronic mail "e-mail" address, and which is provided to the location server upon request; for establishing a VoIP communication session in the WLAN: receiving an INVITE request issued from the gateway and forwarded from the SIP proxy server in response to detection of an incoming voice call from a caller through the telephone network, and thereafter sending a response to the SIP proxy server which is forwarded to the caller via gateway; when switching communication operations from the WLAN to the WWAN: causing a message for forwarding voice calls to the mobile communication device to be transmitted, using the data communication capability via the WWAN, and through the Internet, to the location server of the WLAN for forwarding voice calls directed to the SIP identity of the mobile communication device in the WLAN to the WWAN identification number of the mobile communication device in the WWAN, the message including the WWAN identification number which is received and stored at the location server; and causing a message for cancelling the forwarding of voice calls to the mobile communication device to be transmitted through the WWAN for cancelling the forwarding of voice calls directed to the WWAN identification number of the mobile communication device in the WWAN to the SIP identity of the mobile communication device in the WLAN. operating in the WLAN for communications, which includes: registering with registrar server functionality of the SIP proxy server using the SIP identity which has a format of an electronic mail "e-mail" address, and which is provided to the location server upon request; for establishing a VoIP communication session in the WLAN: receiving an INVITE request issued from the gateway and forwarded from the SIP proxy server in response to detection of an incoming voice call from a caller through the telephone network, and thereafter sending a response to the SIP proxy server which is forwarded to the caller via gateway; registering with registrar server functionality of the SIP proxy server using the SIP identity which has a format of an electronic mail "e-mail" address, and which is provided to the location server upon request; for establishing a VoIP communication session in the WLAN: receiving an INVITE request issued from the gateway and forwarded from the SIP proxy server in response to detection of an incoming voice call from a caller through the telephone network, and thereafter sending a response to the SIP proxy server which is forwarded to the caller via gateway; when switching communication operations from the WLAN to the WWAN: causing a message for forwarding voice calls to the mobile communication device to be transmitted, using the data communication capability via the WWAN, and through the Internet, to the location server of the WLAN for forwarding voice calls directed to the SIP identity of the mobile communication device in the WLAN to the WWAN identification number of the mobile communication device in the WWAN, the message including the WWAN identification number which is received and stored at the location server; and causing a message for cancelling the forwarding of voice calls to the mobile communication device to be transmitted through the WWAN for cancelling the forwarding of voice calls directed to the WWAN identification number of the mobile communication device in the WWAN to the SIP identity of the mobile communication device in the WLAN. causing a message for forwarding voice calls to the mobile communication device to be transmitted, using the data communication capability via the WWAN, and through the Internet, to the location server of the WLAN for forwarding voice calls directed to the SIP identity of the mobile communication device in the WLAN to the WWAN identification number of the mobile communication device in the WWAN, the message including the WWAN identification number which is received and stored at the location server; and causing a message for cancelling the forwarding of voice calls to the mobile communication device to be transmitted through the WWAN for cancelling the forwarding of voice calls directed to the WWAN identification number of the mobile communication device in the WWAN to the SIP identity of the mobile communication device in the WLAN.