A mobile computing device, such as a cellular telephone or cellular network capable tablet or PDA, can be identified by a phone number associated with the mobile computing device. A mobile computing device can include a Subscriber Identity Module (SIM) card that can be used to identify the mobile computing device and to connect the mobile computing device to a communications network operated by the communications carrier or another communications carrier. The SIM card can include memory for storing an International Mobile Subscriber Identity (IMSI). The IMSI can be used to identify the mobile computing device. A computing system can use the IMSI to request device authentication values and/or a phone number for the mobile computing device from a carrier associated with the mobile computing device.