How can I find out who called me if all I have is thier phone number?

Can try verizon.com and do a reverse look-up http://www22.verizon.com/utilities/reverselookup/

do a search on yahoo for reverse phone number lookup. There are a few sites that will search the phone company's database for free.

if you have the number call them back or either don't even sweat it.

http://www.reversephonedirectory.com/

I guess you could just call them, but I've always felt that was a little lame. Either the person who called was too shy to leave a message or it was a wrong number. I take a fair amount of calls at work from people who say, "Uh, your number was on my caller ID. What did you want?" Since this is likely how you will sound to whoever you are calling, I guess you have to ask yourself, "Is it worth it?" If that person wants to talk to you, they'll call back. If they don't, they won't.