In the United States, most cellular companies sell phones at a significant discount when you sign up for a service agreement. And most cellular companies offer discounts on phones when you want to upgrade to something newer as long as you also sign up for another couple of years of service. So, it’s not surprising that most people buy their phones directly from cellular companies. But, What if you did not get your phone from a cellular company?
With a few exceptions, such as an “unlocked” GSM phone, each phone is associated with a particular cellular company. A locked phone can work only on its original carrier. Maybe you bought a second hand phone on eBay, or you got a phone from a friend who didn’t want his anymore.
If you didn’t get your phone directly from a cellular provider, you will need to figure out which provider the phone is associated with and get a service plan from that company.
For example, Samsung Galaxy S phones sold in the United States all have the cellular company’s logo on the phone printed on the front at the top. If there is not a logo on the front, you have a “gray market” phone. These are made by Samsung for sale in an international market but have been brought into the United States.
Either your phone have a logo or not, These phones are perfectly legal, but you would need to figure out which cellular carrier it can work with. The quickest way is to take the phone to any cellular store; the folks there know how to figure it out.
To narrow down the possibilities on your own, you need to do some investigation. The easiest way is to take off the back of the phone to find the plate with the model and serial number for the phone. If you see IMEI on the plate, the phone is based on a technology called Global System for Mobile (GSM); it’ll work with AT&T or T-Mobile (or both). If you see ESN on the plate, the phone will work with either Verizon or Sprint (but not both).