Dealing with telephone harassment starts with maintaining a certain level of anonymity, so getting an unlisted number is a good first step. If you receive a harassing phone call, you should generally hang up the phone to avoid encouraging the negative behavior. If the calls persist, or if they have a threatening quality, don't hesitate to get the authorities and the phone company involved. There are official actions that can be taken by the police, but sometimes it takes more than a single call to get them interested. The same generally holds true for the phone company.
It is helpful to understand the legal definition of telephone harassment so that you know what legal rights you have in different situations. Basically, harassment involves calls that are threatening or intimidating. Sometimes there is heavy breathing or outright verbal abuse, and sometimes the person may just call and say nothing. It has more to do with how it makes you feel than exactly what the perpetrator is doing. Sometimes debt collectors can cross a line in their phone behavior, and it may reach the level of true telephone harassment, but that can be harder to prove legally.
It is usually good to hang up right away, because many prank callers may be encouraged by any kind of reaction. For example, they may find an angry outburst amusing, or they may enjoy someone pleading with them to stop. Of course, hanging up won't necessarily stop them—these people can be persistent, but the more they call you, the easier it will be to get the law and phone company to take your complaints seriously. You should generally start reporting the calls to authorities right away, especially if you feel threatened, but they may wait until there is a clear pattern of behavior before acting on your complaints. If there is an overt threat in the phone call, it is often possible to get some immediate action, but the callers often avoid overt threats for that very reason.
Sometimes even after getting the phone company to set up a trace for you, it may be difficult to track down the offender. These individuals may use pay phones and other methods to avoid detection. In those cases, it could be a good idea to change your phone number and make sure that the new number is unlisted.
One interesting trick to use is to change your number and leave the other number active with an answering machine attached. The goal here is to get the perpetrator to leave threatening messages, which you can possibly use as evidence in a future trial or to get the police to help with your situation. This can be an effective approach for dealing with an ongoing telephone harassment issue.