Apparently, some phone providers lock their phones, meaning that those phones will only work with their carrier's SIM cards.
So if you have an AT&T phone, you can be fairly certain that it will not work if you change your cell provider to T-Mobile or Sprint.
Since many of the wireless carriers provide a free phone or a highly discounted phone they did not want to be out the cost of the phone to find that later that same phone is being used with another carrier, so they got the cell phone manufacturers to design the phone that could be locked to just their service.
To find out if your phone is locked, borrow a SIM card from someone who has a different provider, install it in your phone and see if it works. If it works and you can dial the borrowed party's number and your phone rings, then your phone is not locked.
If it doesn't work, how do you unlock a cell phone? First, don't purchase any of the unlock codes that are on various sites. You can get them for free. To find out how to unlock a cell phone, go to Google or Yahoo and type in your cell phone brand, like Nokia or Samsung or whatever it is, plus free calculator. Nokia Free Calculator.
You may have to do a small download to use the calculator, but you can always uninstall it right after you use it. Once you have downloaded the software, it will instruct you how to find the unlock code.
You'll have to enter in your phone brand (like Samsung), the model (like 1234), and usually the company that provided you service (such as AT&T). You will also need the IMEI code, which you can find on the back of your phone underneath the battery, or by pressing *#06# into your phone. The code will appear on your screen, so be sure to write it down. Do be careful because you only have 5 chances to enter the code correctly before the phone locks down.
If this all seems like too much of a hassle, you can either choose to go with a phone provided or purchased through your new carrier or purchase a cell phone, independent of any carrier, that is already unlocked.
