Global System for Mobile Communications networks make it easier for the user to switch networks, can transmit voice and data at the same time, and are cheaper to build. However, GSM technology uses a less advanced method to handle calls compared to Code Division for Multiple Access systems.
GSM networks store all customer information on removable SIM cards. As such, customers can easily swap phones or networks by putting in another SIM card or putting their SIM card in another device. Because all carriers must accept any GSM-compatible phone, GSM carriers cannot determine what phones the customer can use. In comparison, CDMA networks store customer information on their networks, use lists to verify connections and only use SIM cards to establish a 4G connection. As such, a CDMA network can limit its customers to certain phone models and make it harder or impossible for them to switch to another network.
Because it's a required component of the technology, all 3G GSM networks are capable of transmitting voice and data simultaneously. Although Evolution Data Optimized technology allows 3G CDMA networks to do the same, carriers in the U.S. haven't adopted it.
While an industry consortium develops GSM, CDMA is a product of Qualcomm. As a result, it is much cheaper for carriers to build GSM networks. Historically, the timing of the technology convinced American carriers to adopt CDMA, according to PC Magazine; CDMA was more advanced when the companies decided to update their infrastructure, but GSM advanced faster thereafter.
Whereas CDMA networks can encode call data with unique keys and transmit multiple calls at once, GSM networks must send calls one after another, causing a buzz when near speakers. As such, 3G GSM networks adopted CDMA technology to handle call data.