You will need to contact the previous owner and have them contact their carrier to release the PIN from their BIS account.
__________________ The difference between stupidity and genius is that genius has its limits. When you take things for granted, the things you are granted, get taken. Even a mosquito doesn't get a pat on the back until it starts to work. Too many people miss the silver lining because they're expecting gold. [BES 5.0.3 / GroupWise 2012 HP2]
Try contacting the carrier again and simply quote the PIN of the BlackBerry. They should be able to look up the PIN on the BIS database and release it without needing the phone number. May take a bit of persuading but I have done this for many users before.
I've heard that you may be able to release the PIN by putting in another sim with a Blackberry plan already attached to it. Then that person can log into their BIS admin page and add that device.
Then they can put their sim card back into their phone and log back into the BIS admin page to change the PIN back to their device.
Doing this should release the PIN. This should work for many carriers, but not all from what I understand.