Moved to a more appropriate section.
The 2 year old thread is probably still valid given the age of the 7520. It is almost archaic in cell phone lives.
AFAIK, the 7520 is a Nextel iDen device and can only work on Nextel. Therefore unlocking does nothing.
If you want to use it on Boost, then it might work as is with a Boost SIM, but you won't get any BB services.