The question of how to unlock a cell phone comes up every now and then, especially in posts about prepaid cell phone deals, so I thought it was important enough to deserve its own thread. A quick search showed no existing general threads, although some information about a few specific phone models can be found.

What is cell phone unlocking?
Most cell phone service providers in the US provide cell phones at a reduced cost to their customers in return for signing a contract (of typically two years). In order to minimize the chances of customers switching service providers, they lock the cell phone so that it will work only with a SIM card provided by that company. In order to use that cell phone with another service provider, you must modify the phone's settings so that it will work with the SIM cards of another service provider also.

Can any cell phone be unlocked to work with another service provider?
Currently, there are two kinds of technology in use by cell phone companies: GSM and CDMA. These two technologies are not interchangeable and GSM phones will not work with CDMA based service providers and vice versa. If your phone is a GSM phone, chances are that it can be unlocked to work with another GSM based service provider. However, the difficulty of doing so and the effort and accessories required depend on the particular phone model.

So if I unlock my GSM cell phone, can it be used with any other service provider?
Not quite. GSM phones and service providers are further differentiated by the frequencies they use. Most service providers in the US use the frequencies 850 MHz and 1900 MHz. You can only use your unlocked phone with another service provider if your cell phone is designed to work in at least one of the frequency bands that the service provider uses.

How do I find out which frequencies are used by a service provider?
There is a comprehensive list at GSM World.

How do I unlock a cell phone?
The process varies with cell phone manufacturers and models. For instance, fo most Nokia cell phones, you use the cell phone's IMEI to generate a code which you enter into your cell phone, and this unlocks the cell phone. For other manufacturers such as Motorola and Sony Ericsson, you need a cable to connect your phone to a computer or to special devices which help you unlock the phone.

What is an IMEI and where do I find it?
According to the Wiki: The International Mobile Equipment Identity (IMEI) is a number unique to every GSM and UMTS mobile phone. It is usually found printed on or underneath the phone's battery and can also be found by dialling the sequence *#06# into the phone. The IMEI number is used by the GSM network to identify valid devices and therefore can be used to stop a stolen phone from accessing the network. See IMEI on the wiki.

Are there any risks associated with unlocking a cell phone?
Yes. If it is not done correctly, then there is a chance that the cell phone might be rendered totally unusable. For instance, with Nokia cell phones, most unlocking techniques generate 7 different codes, only one of which is supposed to work. However, you get only five tries, after which the cell phone locks up completely and must fixed by other means. Some people have reported workarounds for this problem or success even after more than 5 tries but it would be wise not to count on it.

Good idea, but I would recommend organizing the quick summary so that is easy to read and follow. Make it as concise as possible in a consistent format.

DCT4 Nokia's should all be able to be unlocked by entering phone info at http://unlock.nokiafree.org/

It would be better to provide links like this instead of links to threads that have this info buried deep in them.

I unlocked my Nokia 3220 and was using with sim from TMOBILE. I used for 3 to 4 hrs when i noticed that it again gave me SIM rejected error. And it is locked again. I received and dialed from this phone using TMOBILE SIM for good 2 hours.
Anyone experienced same situation or explain this?

Thanks

what can you do if its permanently locked?

If the prepaid sim card is from the same cellular company as the phone, you do not need to unlock it.