I found this website (
http://freesms.1888usa.com/revcell.htm) that you can enter a phone number to ID what carrier was the original owner of the number, but like the website says, "...if the wireless customer ported the number to a new wireless carrier, this information will NOT be accurate."
I haven't tried xmentalx's suggestion (teleflip), but I'd be a little weary submitting my associates phone numbers to an unknown source. They could be logging every phone number that goes thru their system and either using it for personal marketing or selling it to 3rd party marketing firms. I know, call me paranoid...
I guess until the MMS issue gets resolved, this might be the closest resolution we have.