Old phone...

So I realize that my old phone battery is the exact same as the battery my new phone uses. Decided having two batteries fully charged would be a great idea. Turn on my old phone, which was with Nextel, and now I'm on Cingular. Crapular. But I see that I have two voicemails with my old phone number. Problem is that I have no idea how to get them. Don't have service anymore. And I don't remember my old cell number. Could be important voicemails. Maybe not. But I'm wondering...