Did you buy this phone used? It sounds like the PIN number might have previously been registered to someone else. If this is the case, the person you purchased the phone from needs to contact their service provider and request that the account be deleted so that the PIN is released. You can then call your service provider and have the PIN registered to your phone.
If it is a new phone, call your service provider's customer service and ask to speak to the Blackberry Data Support team. They can assist you with getting the account provisioned properly.
If I knew where I was going, I might already be there. -- Cross Canadian Ragweed.