mobile phone number portability

It has been few years since people are able to switch their mobile carrier without changing their mobile phone number. I recently got a Blackberry 8830 from Verizon Wireless. My old number was a part of a family plan. My new phone is a part of the Temple business account plan. And, I was told that I am not able to keep my old number. It seems really strange when they can let their customers who are switching over from another carrier keep their old number, while cannot let their own customers keep their old Verizon number. Something is not quite right in this picture.