Caller ID revolutionized how we communicate by phone. When it debuted, people no longer had to answer the phone to know who was calling. But soon after, call blocking was invented to prevent Caller ID from identifying the caller's number. If, however, you're receiving harassing or threatening phone calls from a private number, you need to be able to trace it and find out who's behind the private number. There are a few ways to accomplish this, and the method you choose should be proportional to the severity of the calls.
Sign up for a service called TrapCall at trapcall.com. When a person using a private number calls you, press the "Ignore" button on your phone and their number will be redirected to the TrapCall database. TrapCall sends you a notification, telling you the number. There are a few levels of service beginning at $4.95 per month. The basic service unblocks blocked numbers and allows you to blacklist numbers as well. See the link in the "Resources" section for more details on plans and pricing.
Call your network provider's customer service line and ask for the "annoyance desk." Explain that you're getting a multitude of harassing calls or prank calls and you want to find out who's behind the private number. They can set up a Call Trace on your phone line. You may incur an additional fee, depending on the nature of the calls and your service provider's policies. After signing up for the Call Trace service, dial "*57" after you receive a private number call, and the number should be revealed.
Contact your local police department. If the nature of the calls are threatening to your safety or your property, it's a legal matter. This is typically a last resort, but if you're receiving extremely harassing calls, the police can put a trace on your phone line and find the number behind the "Private" ID. Don't delay contacting authorities if you suspect your safety is in danger.