Phone security code is 12345 as default. If you've changed it to something else you're out of luck. If you're not talking the security code, but rather the SIM lock code that's a whole other matter.
Nokia 5610 Unlock Codes To view Software Version : *#9999# *#92772689# for checking ISDN number. * Serial No. * Made * Programming date * Repaired * Transfer user data ? * Life Timer *#639# to change NAM Free SMS Tip * Turn your phone off. * Turn it on and enter your PIN * enter *#746025625# * now "SIM CLOCK STOP ALLOWED" will be displayed. Now, you can send SMS for Free !. If you turn your phone off and later on, you have to re-enter this code.
You looked this up on the net didn't you? For one thing GSM phones don't have a "NAM" Also, the free SMS "tip" is bogus.