Patent ID: 7027813

Claim:
A method of determining whether a mobile phone is currently in a home network or a roaming network, the mobile phone including a SIM card resident thereon, the SIM card including an IMSI file, an OPL file, and a PNN file, each file having a record structure, the method comprising: receiving, in the mobile phone, an over-the-air signal comprised of control data including public land mobile network (PLMN) data and location area information (LAI) data; comparing the received PLMN data to PLMN data stored in the IMSI file on the SIM card, and if the received PLMN data matches the stored PLMN data, then determining that the current network is a home network; otherwise checking whether the received LAI data is contained in the OPL file, and if not, then determining that the current network is a roaming network; otherwise checking if the PNN record that the OPL record points to is the first record of the PNN file, and if it is, then determining that the current network is a home network; otherwise determining that the current network is a roaming network.