My nokia n73 show message memory full delete some data

Some phones can be unlocked by modifying their software, while others require an alteration to their hardware. You can ask your carrier about unlocking your phone, but it's not likely that they'll do it--especially if you're still under contract. Alternatively, you can pay a third party to unlock your phone for you, but remember that if your phone is damaged, you're not likely to get any help fixing it. Unlocking it probably voids any warranty you may have.

And remember that it may not make sense for you to unlock your phone until your service contract has expired, anyway. You'll either be forced to pay a monthly fee for the remainder of your contract, or you'll have to pay a termination fee to break your contract.