Although you might have caller ID installed on your phone, you can still receive calls that come up as "Unavailable" on your call log. This can be especially annoying if you get multiple phone calls from the same anonymous number. Assorted tools allow you to trace an unavailable phone number and find out who is calling you.
Use a reverse phone number directory to find out who is calling you from an unavailable number. On some caller ID systems, you can see the caller's phone number but not her name. To trace this type of call, place the phone number in one of the many reverse phone directories found on the Internet.
Communicate with others to find out the identity of the person or company making anonymous calls. On 800 Notes, you can post a forum entry about a particular anonymous phone number. Other users familiar with this number might know the identity of the caller.
Type the unavailable phone number into an online search engine such as Google. Type the entire phone number including area code to see if an address or business name shows up in the search results.
Install a privacy manager on your phone. This extra service is offered for a fee by many phone companies. When you have this service, callers with anonymous numbers must identify themselves before the call will go through to your line.
Report the unavailable phone calls to your service provider. They may be able to trace the calls on your behalf. For example, Sprint offers Malicious Call Trace for harassing phone calls. They may charge a fee for this service.