Here's what you should do. First, call your phone company, and tell them exactly what is happening. Phone companies are required by law to take steps to protect their customers and/or cooperate with the police when someone complains about being stalked. (This is for landlines only, cell phone carriers aren't subject to these laws.) Get a confirmation of one sort or another that you've reported the problem. Then go to your local police station, and tell them everything that is happening. The police may be jerks at times, but this is the situation where their help is critical. Most likely, they'll contact the person who's making the calls (your phone company will trace the number), and demand them to stop calling.
I had a similar situation, except I was stalked by someone I used to be friends with. We had a falling out because he was blaming me for something I had no part in. During the time he stalked me, he was calling my home phone up to 20 times a day, leaving threatening messages. So I made a recording of all the messages the "friend" left, burned it onto a CD, and took it with me when I went to the police. The officers at the station complimented me on being smart enough to record everything like that. After they listened to the whole CD, they said that the "evidence is strong enough to make an arrest" (their words). Next thing I knew, I saw two squad cars pulling out of the station. I don't know what happened afterwards, but I never heard from the guy again.
In the light of that, screen all your calls. Let the caller leave messages on your answering machine. If he doesn't do that, answer the phone and listed to the voice. If you hear heavy breathing, immediately say that you'll be recording the call. (In many states, you can't record a phone call without notifying the other person, and any response except "no" is considered consent.) In both cases, use a voice recorder or a microphone hooked up to your computer. Burn the recording onto a CD. Then take it to the police.