GSM is one type of digital technology and is not compatible with CDMA, TDMA or iDEN. A SIM card (subscriber information module) is a removable account card and has a serial number unique to it that identifies you to the wireless provider. A GSM phone is not directly linked to anyone. This means that you can put your SIM into any compatible phone and that phone will now act as yours. It identifies to your SIM not your phone. So now you can simply switch providers without switching phones unless the provider has their phones programmed or "locked" to work only with SIM's issued by their company.

In the United States, you can use either a phone with either T-Mobile or AT&T, the only current GSM wireless providers. If you travel abroad, you can swap out your SIM card for a foreign one.

It is a relatively easy process to get service for an unlocked phone. First order and sign up for service either at a T-Mobile or AT&T store or on the carrier's Web site. After you have your SIM card it is relatively simple, just place it in the phone and start making calls.

How can you check if your phone is unlocked or not? It is as easy as placing a SIM card from another phone with an account from a different wireless provider into your phone. If it works and shows the other phones number then the phone is unlocked. If you get an error message or it will not work then the phone is locked.

Please keep in mind, though that in order to browse the web; you will need to make sure your phone is programmed with the Internet settings unique to either AT&T or T-Mobile. The carrier can give you the settings or you also can find them on the carriers' website.

Why would you want to unlock your wireless phone? You want to change your current wireless provider in the US and still keep your phone. You changed your provider and got a new phone but want to use the other as a backup by swapping the SIM if you experience problems.

You are going to travel internationally and you don't want to pay the higher international roaming rates.

How do you unlock your phone? There are basically two ways. Your phone can be unlocked by simply entering a code into it. The code is a unique number for that particular phone.

Some types of phones need their operating software to be rewritten to remove the lock. This means connecting the phone via a data cable to a special programming unit. There are professional unlocking services that will provide this service for a fee.