Methods and systems by which a client application program running on a mobile communications device can determine the telephone number associated with the mobile device and communicate the telephone number to a remote computing entity in a secure and verifiable fashion are presented herein. In one embodiment, a client application program running on a mobile device sends an SMS message to a remote server, allowing the remote server to determine the telephone number associated with the mobile device and return the determined number to the client application program in a SMS response message. A client application program running on a mobile device may also prove to a remote computing entity that the program is running on the device. The client program and remote entity may exchange information in SMS messages, and the client program may transmit information obtained from the remote entity to the remote server via WAP-based communications.