In the past, the US wireless market was split into GSM and CDMA service technologies with two carriers supporting each of these technologies. Modern smartphones support LTE and 5G service without CDMA technology your choice of smartphone is not as limited.The most popular smartphones in the US, Apple iPhones and Samsung Galaxy phones, support all wireless technologies in the US wireless market so there are no limits on using these phones with any US carrier. If you purchase these phones directly from Samsung and Apple, then you can choose to purchase unlocked models.Some phones are sold in a locked state so they can only be used on one specific carrier. This may be changing soon and there are processes to unlock your phone from your carrier if you decide to switch carriers. There used to be incentives for purchasing locked phones, but in today's world purchasing unlocked phones gives you the most flexibility for choosing your service provider.