In an overlay Wireless Location System, an Abis interface is monitored to obtain information used to locate GSM phones. Signaling links of the Abis interface are passively monitored to obtain certain information, such as control and traffic channel assignment, called number, and mobile identification, which is not available from the GSM air interface of the reverse channel. This approach also applies to IDEN and can include CDMA systems where the GSM architecture has been used and the system includes a separated BTS to BSC interface.