Unlock Nextel I60c How Do I Unlock A Motorola I60c To Use On Another Service?

How do i unlock a motorola i60c to use on another service? - unlock nextel i60c

Does anyone know the code or codes to unlock a Motorola i60c phone cover, so you can insert another SIM card is in it? It currently has Nextel SIM card and SIM card I use att.