Ther is no directory for cell #'s as of yet. IF their old # is no longer active they probably went to another service provider - no chance the old Co. would have a clue.
cell phones are not listed anywhere - this goes along with the laws that say telemarketers are not allowed to call them. Since most people have low usage plans, they could be paying by the minute for calls. Cell phones are all unlisted.
So your "friend" changed their number, and won't give you the new one....
http://www.reversephonedirectory.com will tell you the address of a person when you have the phone number (don't know about cells though). Perhaps you could start there to find the address.