Using reverse cell phone services to lookup cell numbers can provide you the ultimate ways to find information on anyone you choose, including information on the person who has been calling your mate. The most commonly used search engine across the globe is Google - hands down.
Many will call their phone company when they have a problem, and that can be a great way to start. This is why you need more information about the cell phone number and this is how reverse cell phone lookup comes in. The subject of weather or not cellular lines should be considered public domain or not has been discussed vigorously as of late.
This means that a number may be coming up over and over again, but you really have no clue as to who it might be. I have seen websites out there that claim to not to charge but you end up paying in the end. You can use reverse information sites when either of these are a problem.
Don't be afraid to take advantage of what is out there. Get out there.