Originally posted by: Azndude51
I'm pretty sure Nextel uses GSM. All GSM cell phones are designed to be used by any service provider, as long as you have a valid SIM card with the provider. A locked cell phone means that it can only be used by the service provider who released the phone. For example, if I placed my friend's Cingular SIM card into my T-mobile phone, the phone wouldn't work because it's locked and can only be used with T-mobile. However, if I unlocked it, it will work with his Cingular SIM card (with Cingular's service.) You shouldn't worry about unlocking your phone unless you want to change service providers or take your phone to another country and use a foreign SIM card.
This is a pretty good forum to find out info on how to unlock your phone, many models do it differently.
EDIT:
I had my phone unlocked when I went to China. It's much cheaper to use a Chinese service provider's SIM card/service there than use the $2 a minute T-mobile long distance service. For my Samsung, I had to get a data cable and download new, non-T-mobile branded software.