Let's first talk about why you are able to unlock your GSM phone. A GSM phone utilizes a sim card to store your account information that enables the service provider to confirm your identity. The great thing about this, is that if you decide to switch to a phone that is offered by your current service provider you are able to do this quickly and effortlessly by simply removing the chip located behind the battery and placing it into your new phone. The difficulty begins when your current GSM service provider does not carry the phone you want.
So it begs the questions.
What do I have to do to get the phone I want to work with the service provider of my choice?
Answer: You are going to have to unlock the phone before the phone will accept the sim card.
Unfortunately all carriers lock their phones, meaning their cell phones have been programmed to work with only the sim cards the companies? issue. To get around this you are going to have to either purchase a phone that is already unlocked or find a reputable website that can unlock the phone for a reasonable fee. If you have a Nokia phone your in luck. Most Nokia phones can be unlocked remotely by entering a special code. The code, along with the detailed instructions is usually emailed to you immediately after you complete the checkout. Other phones, such as the Treo 650 need the phones? firmware reprogrammed. This is not something that can be done remotely and will require you to send your phone to the company. I would suggest doing a local google search when looking for someone to unlock your phone. You may get lucky and find someone close enough to simply bring the phone to their location.
In any event, it is a small price to pay for such a convenience.