Your old company is letting you keep their cell phone? Was it yours and they just reimbursed you for it?

If you can't port your number to the new cell company phone, I would change your voicemail message on the old work phone to say "call me at my new number" and disable the ability to leave messages. Drop it to the minimum contract price for a couple months, then cancel it if you can. Numbers get changed all the time - people will adapt.

Keep the blackberry for personal use.