GSM (AT&T, T-Mobile) vs. CDMA (Sprint, Verizon)

GSM and CDMA are two different technologies that are competing in the US. AT&T and T-Mobile are the major players using GSM, while Sprint and Verizon are the ones using CDMA.

There is a huge difference in how SIM cards work between CDMA and GSM. With GSM, the subscriber data is stored on the SIM card. This means you can pop it into any phone with a SIM card slot and it'll just work without needing to even contact your carrier. With CDMA, subscriber data is stored with the carrier. This means to switch a phone over to a different CDMA carrier you'll need for both carriers to work together in swapping over your data, or to use a different phone with the carrier you need to go through them and have them pre-approve it. The reason CDMA carriers use SIM cards at all is because they were forced to adopt it when 4G became a big thing.. It is not even close to what SIM cards do with typical GSM carriers. What's the problem? Well, the majority of the world uses GSM as a global standard and a lot of phones are not built CDMA compatible because the USA is one of the weird cases where CDMA is popular. It's a lot easier for an OEM to build a GSM-compatible phone and just let the user swap a SIM card in and have it work. This is why you normally don't see OEM's selling two unlocked versions of their phones, CDMA and GSM versions. Because CDMA carriers need to allow and support unlocked phones, while GSM carriers can just hand out a SIM card. Recently there have been unlocked phones released for CDMA thanks to popular demand, but to get an unlocked phone working on a CDMA network you still have to get it activated with the carrier instead of just swapping a SIM card. Besides, most of the world uses GSM anyway.

Image source: PC Mag