You will often hear people suggesting to use Google or other search engines to do a reverse cell phone number lookup.
While you can certainly try it, you will most probably see that it often isn't the easiest and certainly not the most effective way to look up a cell phone number.
The reason for this is that Google (and other search engines) can only index what's out there on the Internet.
The chance that the owner of the cell phone you want to look up has published his private number somewhere on the Internet is pretty low.
Nowadays, most people are really careful to not publish too much private information on the net.
You will find that the most effective way to find out that number you're looking for is using a professional reverse cell phone lookup directory.
These directories have paid for the millions of cell phone numbers they have in their databases, so the chance to find the number is pretty good.
The only small drawback is that you have to pay a very small fee to access a full report.
If you just want to be nosy it's probably not worth it, but if you are getting prank or threatening calls or suspect your partner is having an affair, the small fee is definitely justified in my opinion.
You can do a free search to see if they have got the number you're looking for before you have to decide if you want a full report including the owner's name, his address, an address history, and more.