If someone's online cell phone number is found, it's very similar to finding a needle in a haystack. This is because mobile phone numbers are protected by privacy laws and mobile phone records are restricted by privileged organizations and individuals. However, these rules have changed. Recently, some mobile phone directories have opened up the public. Now anyone can search cell phone numbers without having to go to an expensive private detective.
Finding such public cell phone directories can be as difficult as finding a cell phone number yourself. This is due to the fact that mobile phone directories are new. You will run countless reverse cell phone search sites. But these are useful only if we look at or check phone numbers.
There are many misleading and spam web sites that just pretend to want the information you want. These scam sites only exist when they try to figure out the ads. Many of these ads lead to similar sites and reverse phone directories.
Another type of mobile phone number library is just someone's number when sent to your own site. This is an extremely handy way to find someone's mobile number, and I doubt that you would ever find a number of people unless they were told first.
The other site is National Mobile Number Registrar at http://www.cellphoneregistry.org which is great because any number can be searched for by the US and Canada. The best part of this site is search for court records, criminal records and other public records that most people do not even know about.