The free lookups are available for ordinary land lines as well as business phone numbers. Those phones can be easily found with no charge for example at WhitePages.com website. They claim to offer over 400,000,000 of residential records absolutely free of charge. This means that if you type a phone number and it's found in the database, you will be able to find out the owner's name as well as his home address. However, there are phone numbers which are actually landlines but cannot be found in the public phone directories such as WhitePages. Those numbers are called "unlisted" and their owners requested their data removal from public sources. This is very common practice which is intended to protect their privacy. Although you can't find their information in public databases, you can try to employ paid backwards phone look ups to find more details about those numbers.
So if you need to reverse a number which hasn't been found in for example WhitePages, you will have to check paid services which have much bigger databases. The data for those directories is collected in totally different way and since residential records are public information, you can just ask a phone carrier for the data of his customers, but cell phone numbers as well as unlisted cannot be handled this way. Those data brokers have to ask each and every cell phone owner if they allow them to list their information in their database. Of course this is a simplification and the process is much more sophisticated but according to the current US law the company which collects such data must receive a consent from each and every person.
If you still can't find the record which you need, you can try to employ a private detective. And although this is the most effective method of find the phone number's owner, it's also the most expensive one I would advice it only to people who are really desperate to find out somebody's identity. The fees for a basic service will start at at least $300 and they might raise to even a few thousand dollars, so the costs can be enormous. However, you are assured that the information you receive is accurate and double checked.