The first thing to do is note down the time and day of the first call. Then, you need to immediately dial *69 on your cell phone. This will allow you to call back the last number that you received on your cell phone. The number you call won't help you find the other caller's number, just help you speak to the person. You can then try to find the person's identity as most of the time it is someone you know who tries to harass you.
If this does not work, then you need to activate call trace on your phone. You can do this by calling up your service provider and asking them to activate the service. You will usually be charged a fee by the network provider for activating this service. Once activated, you can dial *57 and track the number that called you. But, this is a long treacherous process, as you will need to call the service provider to track the number then they will forward it to the police, if you have filed a complaint. Then, you will need to request the police department to provide you with the details of the tracked number.
You can even try to activate blocking private call service. Any anonymous calls or calls from private numbers will be blocked on your cell phone. This way you will not receive calls from blocked numbers. The service provider may charge you with a certain amount of fee for this service.
You need to note down the number of times you get the call, the time of the day and what is being spoken on the phone. Even if the person does not say anything and just remains silent or breathes heavily, it amounts to harassment. You should file a complaint regarding these blocked calls and ask them to look into the matter. The police will trace a blocked call and issue warnings to the offending party. If necessary, charges may also be filed against the person bombarding you with these harassing calls.
Many times, someone just tends to make harassing phone calls for fun as they have nothing better to do in life. The best thing to do is hang up on the caller without saying anything. If you get angry or express your discomfort, you are just giving them a chance to trouble you more. If this doe not work, set up a voice mail system. Whenever someone calls, the recording for voice mail should say something on similar lines:
I am sorry I cannot receive the call right now. Please leave a message as we are receiving harassing calls. For this reason the phone company has set up a trap on this line. If no message is left, it will be assumed this is an harassment call and the call will be traced by the police.
People will definitely leave a message and the caller harassing you will have no choice to end his/her mischief. Suppose you receive the call, all you need to say, Operator, this is a harassment call and hang up immediately. This may help deter mischievous callers from making prank calls.
You need to make sure you call the network provider and contact the police, if things worsen. Changing your number can be an option, but if you are facing this problem often, you should contact the police.