unlocking a locked sony ericsson s5001?

i need help i found a phone but the phone is locked meaning as soon as i open it up i have to have a 4-8 digit password to get in the phone.i was wondering if theres any way to unlock the phone or resetting it so theres no password. i tried putting the sim card in my phone ( both are for tmobile) and press master reset but the sim isnt locked the phone itself. idk wat to do now and i cant call any carrier cuz it aint my phone. plzz give anyhints on unlocking this phone other than trying every number in the world or bringing the phone back to owner.

Make sure that your Sony Ericsson is locked with your present carrier . To confirm it, just insert another network sim card in to your phone . If it asks for unlock password, you can found that your phone is locked with your network not with security lock . To unlock it , use unlock code . You can get it from any of the online vendors like TheUnlockSpot.com to unlock it .