The use of SIM cards is almost ubiquitous among major cell phone carriers in the United States as of 2015, largely due to the widespread adoption of 4G cell phone technology. However, AT&T and T-Mobile make use of SIM cards in almost all of their phones, including older models.
Older cell phone technologies utilized either the GSM or CDMA standards for handling cell phone transmissions. These are mutually incompatible technologies, partially due to the use of SIM cards in GSM phones to contain the data for connecting a phone to the cellular network. In contrast, CDMA phones encode this data within the phone itself. Cellular service providers AT&T and T-Mobile chose to develop GSM-based networks in the 1990s, as did cellular providers in most of the rest of the world, while Sprint and Verizon Wireless chose CDMA technology. Older phones using 3G or earlier cellular technology therefore have SIM cards if designed for AT&T or T-Mobile networks, but no SIM card if intended for use on Verizon or Sprint.
Due to the popularity of GSM networks in most of the world, 4G cellular technologies were developed using a base of GSM networks. Because of this, 4G-compatible phones used with any cellular network must have a SIM card slot. However, Verizon and Sprint 4G phones also have CDMA hardware in addition to SIM cards to enable compatibility with the 3G cellular towers still in use by those companies.