I too have been bombarded by calls (and no voicemails) from this University of Iowa number. I finally answered one, just to see if there was a real person on the other end who I could ask to stop calling me. It was someone from career services doing some sort of exit survey-- they wanted to know what UI alums were doing education/employment-wise, and wanted to tell me about the career services they offer to alums. I told the first part, and declined to hear the second part. The young person (probably a student) who I talked to seemed polite, and like he was just doing his job. Hopefully this is the last I'll hear from them.
My advice: if the constant/annoying calls really bother you (like they bothered me), answer once, and ask to be removed from their list. This took a whole 30-45 seconds, versus the minutes wasted checking my phone over and over to see who kept calling.