For heaven's sake, call the phone company!
If someone is using the telephone to harass you, all you need to do is request them to help you track down that person. The phone is yours, and they are not only legally obligated but generally eager to help victims of telephone stalking.
After the pattern has been established, that person's own phone service can be -- and usually will be -- terminated, and you have the option of pursuing both criminal and civil complaints.