A method and a system for wireless terminals registering a communication system, comprising wireless terminals, public land mobile network (PLMN), wireless local area network (WLAN), Wherein the wireless terminals are able to communicate with both the PLMN and the WLAN. The PLMN coverage overlaps the WLAN coverage linking the two together. First, the wireless terminal obtains an ID, attaches to a PLMN, and then transmits the ID to PLMN to authenticate and register. After authentication, the PLMN generates an authentication code and transmits the code to a wireless terminal. Additionally, the PLMN transmit the authentication code to at least one WLAN. The wireless terminals must communicate with at least one WLAN.