The only thing I can think of to advise is to change your phone #. I know that's annoying and inconvenient for you, but there is probably no other way to really stop the calls.
Speaking odd calls, I was in the grocery store about a month ago and I noticed I had missed a call from my kids. I checked my voicemail and it said I had 11 new messages. Oddly enough, I had only missed 1 call. So, as I am walking through the grocery store, I listened to the messages. Of course 1 of the messages was from my kids, but the other 10 were exactly the same message... it was someone playing me a "spoof" and somewhat nasty version of The Thong Song... over and over, 10 times. And even odder was the messages were all at different times and dates. They were spread out over 3 days and some were during the day, some were in the middle of the night. But when I checked my call history, there were no calls during those times. I called Sprint and they also showed no record of any calls during any of the times the messages were left. I even took my phone into a local Sprint store and had them listen to the messages and check out my phone. When you listen to the messages, they told you what number they were from but when we tried calling that number, it was not in service. Even Sprint was stumped. They have no idea how that happened. I never did find out, but luckily it hasn't happened since.
Sorry to steal your thread here.... just when you mentioned prank calls, it made me think of my experience. If the calls continue and really annoy you, I'd call and ask them for a new number.