Generally your cell phone service provider wants you to stay with them as long as possible, so they lock your phone to prevent it from being able to be used with other providers. The lock function comes in and prevents it when you try to switch companies but take the same cell phone with you.
Some cell phone brands however, can be unlocked if you choose to switch providers after all. One downside to cell phones having the ability to be unlocked, is that thieves rely on that chance that will be able to use Nokia lumia unlock codes and use or resell a stolen phone.
There are two main cell phone technologies used overall, a CDMA cell phone which can not be unlocked no matter what, but a GSM cell phone which has a SIM card inside can. You know how important a SIM card is if you have ever had one lost or damaged, it is pretty much everything on the cell phone you might need including your contents and your cell phone number which makes it possible to switch cell phones at leisure when you can access it. A SIM code can be locked if the card and the cell phone were told not to function in other situations if they were separated.
You might want to use Koodo unlock codes if you are planning to travel abroad and want to take a prepaid SIM card or you just want you switch service provider companies.
Unlocking a phone usually is not too hard and your service provider might even be up for it if you pay them. There are also outside service companies that unlock cell phones for a reasonable fee. If you want to unlock a smart phone that can get a little more tricky and is called jail breaking because the process is more intense. Things can go wrong which would leave the phone not working anyway.
Laws about unlocking cell phones and AT&T unlock codes by country and it is your responsibility to be informed. In most places it is legal to unlock a cell phone but it can break your service provider contract to make sure that it something you are prepared to deal with. If you decide to use an outside company to complete the unlocking process on your cell phone make sure you find one that has a good reputation or has been successfully used by your friends. The goal is to end up with a phone that works better for you, not one that does not work at all!
Cell Unlocking Services 