TracFone Wireless, Inc. is a prepaidmobile virtual network operator in the United States, Puerto Rico, and the US Virgin Islands. TracFone Wireless is a subsidiary of Mexico’s largest telecommunications company, América Móvil, and offers products and services under several other brands. It operates as a mobile virtual network operator (MVNO), holding agreements with the United States’ largest wireless network operators to provide service using their networks, including Verizon Wireless, AT&T Mobility, T-Mobile US, Sprint Corporation, and U.S. Cellular. TracFone Wireless had 25.668 million subscribers in 2015. Originally, TracFone service was limited to TracFone-branded phones, which are locked to the TracFone service using an internal SIM card. Other GSM phones, even those that were unlocked from another carrier, could not accept a TracFone SIM card, because these are bound to a specific handset. However, in 2013, TracFone unveiled Bring Your Own Phone, selling SIM cards that could be inserted into qualifying non-TracFone phones (such as Verizon CDMA phones) to register them with TracFone. In 2015, this program, renamed “Bring Your Own Device”, was extended to compatible unlocked GSM phones. TracFone sells phones by many manufacturers. Each phone contains a TracFone SIM card. TracFone sells phones for several networks; models using the Verizon or Sprint CDMA network have the letter C in the model name, while phones using the AT&T or T-Mobile GSM network have the letter G. The choice of phone thus determines the network used and the quality of service in specific locations. A typical retail display includes a coverage map (or several, one for each relevant network), indicating the parts of the U.S. where the phone can be used. The customer must also buy a TracFone service card. These are available where the phone is sold, and at many convenience stores and pharmacies. A typical service card carries a specified number of service days and usage units (such as minutes). Each service card contains an “Airtime PIN” beneath a mask that the customer scratches off. The customer can use a website, use TracFone Customer Service, or send a text, to redeem the card. Redemption loads the phone with the resources specified on the card, and prevents the PIN from being reused. The service card has a bar code that is scanned on purchase, so that PINs from stolen service cards never become valid.