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.