How to Unlock a cell phone, What is Unlocking technologies ?
A handset can be unlocked by entering a special code, or in some cases, over-the-air by the carrier. Typically, a locked phone will display a message if a restricted SIM is used, requesting the unlock code.
For example, on the Sony Ericsson T610 mobile phone, "Insert correct SIM card" will appear on the phone's display if the wrong SIM is used. Once a valid unlocking code is entered, the phone will display "Network unlocked". In some cases, the phone will simply display a message explaining that it is locked. This is especially the case with handsets provided by AT&T Mobility.

The code required to remove all SIM locks from a phone is called the master code or network code key.

The unlock code is verified by the phone itself, and is either stored in a database or calculated using an obscure mathematical formula by the provider.

Newer Nokia phones have more robust encoding algorithms and permit fewer attempts at unlocking and are not unlockable by these free unlocking programs.

Many other manufacturers have taken a more cautious approach, and embed a random number in the handset's firmware that is only retained by the network on whose behalf the lock was applied. Such phones can often still be unlocked, but need to be connected to special test equipment that will rewrite that part of its firmware where the lock status is kept.

Most phones have security measures built in its software that prevent users from entering the unlock code too many times, usually four. After that the phone becomes "hard-locked" and special unlocking equipment has to be used in order to unlock it.

Handset manufacturers have economic incentives both to strengthen simlock security (which placates network providers and enables exclusivity deals), but also to weaken it (broadening a handset's appeal to customers who are not interested in the service provider that offers it). Also, making it too difficult to unlock a handset makes it less appealing to network service providers that have a legal obligation to provide unlock codes for every handset they've ever sold.

The main reason to unlock a phone is to be able to use it with a different SIM card. For example, when traveling abroad it's usually cheaper to temporarily use a foreign network, for example with a prepaid subscription. Contrary to some beliefs, an unlocked phone can't access extra cell phone towers or give free phone service. All it can do is accept other SIMs.

In some cases, a simlocked handset is sold at a substantially lower price than an unlocked one, because the service provider expects income through its service. A consumer may choose to unlock the phone and continue using his previous provider. Therefore, simlocks are usually employed on cheaper (pay-as-you-go) handsets, while discounts on more expensive handsets require a subscription that provides guaranteed cash flow.

A practice known as "box breaking" is common in the UK and some other markets. This involves purchasing (usually) pay as you go handsets from retail stores, unlocking the phones, and then selling them (often abroad) for a higher price than the subsidized retail price. The SIM card that came with the subsidized handset is then either thrown away or sold or used elsewhere. This practice is entirely legal in the UK, and provides a de-facto limit to the extent to which networks are willing to subsidize pay as you go handsets. In recent times network operators have been insisting that new customers purchase substantial amounts of airtime at the same time as they buy a new handset, in order that the total price they pay comes close to the true value of the handset.
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Unlocking via code
This service requires that the individual who wishes to unlock their phone emails his or hers IMEI number (you can find this by pressing *#06# on your phone) to the company. The company will then process this IMEI number and email back an unlock code and instructions. Input the unlock code and your phone is unlocked. These email services are usually the most efficient as it is the same method most retail stores will offer.