I bought a 2nd hand

I bought a 2nd hand Motorola Hellomoto cellphone and when I went to unlock it I wasn't sure of the security code. Well after trying 3 times it locked the phone to where it won't make calls-it says unregistered SIM Cellular One. Also after 4 tries I managed to remember the code and unlocked it as far as settings, messaging, etc. How do I get it unlocked? I have the SIM # but not sure I remember the phone number of the phone! I heard of the codes you can buy-are there any free ones or cheap ones?