Apparatus and methods are described, including an interrogation device comprising an antenna, a transmitter-receiver (transceiver), and a processor. The processor, via the transceiver and the antenna, solicits a mobile communication terminal to associate with the interrogation device. While the solicited terminal is associated with the interrogation device, by communicating to the terminal respective identifiers of a plurality of base stations that collectively belong to a plurality of mobile communication networks, the processor drives the terminal to receive respective signals from at least some of the base stations, and communicate a property of at least some of the received signals to the processor. Other embodiments are also described.