A mobile phone includes a mobile communication module and a wireless communication module. The mobile communication module registers with a session initiation protocol (SIP) server by a first Internet protocol (IP) address and a voice over Internet protocol (VoIP) account over a mobile communication network, and enters a suspension mode when the mobile communication module has no communication. The wireless communication module registers with the SIP server by use of a second IP address and the VoIP account over a wireless communication network, and enters a suspension mode when the wireless communication module has no communication. The wireless communication module receives an SIP invite from the SIP server over the wireless communication network and wakes up the mobile communication module when a remote phone calls the mobile phone. The mobile communication module transmits an SIP response to the SIP server over the mobile communication network after being waked up.