Someone has been kidnapped. Will the robbers let her use her mobile phone freely?

Of course, the robbers will do nothing. They can use their mobile phones freely, which means they can send short messages, visit QQ and make phone calls. At this time, the kidnapped person will definitely call the police, and the police will definitely use GPS to locate the victim. In order to determine the identity of the victims, since they want to get the money, how can the robbers do so?