Cell phones that are purchased from wireless providers are automatically "locked" to that network.  What that means is that if you decide to switch to a different network/carrier, you will be unable to take your phone with you to the new network, despite the high costs you might have paid for it originally.  The better option is to purchase an unlocked cell phone in the first place, which will help you avoid this potential problem and also comes with some other benefits.

Unlocked cell phones generally cannot be purchased from wireless providers and must be purchased from a third party.  Some of the benefits of purchasing an unlocked cell phone are:

1.  You will not have to lock in to a contract with your provider.  Carriers offer phones at discounted prices with the stipulation that customers usually have to sign a contract that locks them to the providers network for 2 years.  If you want to switch, a steep early cancellation fee awaits you.  However, purchasing an unlocked cell phone allows you to have a phone that is capable of working on multiple networks, allowing you to avoid signing any contracts and making it possible to easily switch and change carriers if you want.

2.  You can usually find more phones available when looking at unlocked phones.  Cell phone providers' stores usually carry 20-30 phones that they make available to their customers.  The unlocked market is literally full of hundreds of phones, meaning you can essentially get any phone you want with any feature you want.  The only thing to be aware of here is that your carrier of choice has a network capable of using all the features available on a particular phone.

Unlocked Blackberry Pearl

3.  An unlocked cellular phone can allow you to avoid big international roaming charges should you travel overseas.  Many people that travel internationally a lot understand that roaming fees are extremely expensive for calls placed overseas.  Some travelers have taken to purchasing prepaid cell phones while overseas to avoid these fees.  However, an unlocked cell phone makes it even easier as you can simply purchase a prepaid SIM card for the international network you need to operate on, place it in your phone when you arrive in the country, and use the phone like normal without paying the roaming fees as you will be operating on the network in the country you are in.

4.  If or when you decide to upgrade your phone, you will have much more luck selling a used phone if it is unlocked.  Many people have no idea what to do with old phones and end up keeping them for emergencies or other uses, however if you have an unlocked phone, you may find that you can sell it a get a bit of your money back.

Unlocked cell phones provide their users a lot of freedom and peace of mind as they are not locked in to contracts.  However, you must be aware of where you purchase your phone from.  Try to stick to reputable dealers when purchasing an unlocked phone.  A good way to measure the credibility of a seller is whether or not they offer a warranty on their unlocked phones.  If they do, you should have much more confidence buying from them.