Multiple calls, no messages left, yet no one on the line when you answer the call.
1) These calls are from a phone center, from investigators who have been contracted to conduct government surveillance or surveys regarding who does/does not have health insurance -- i.e. a type of invasive spying
2) Identity theives using VOIP calling to barage people with calls, to "Phish"
3) Other intrusive and illegal surveillance efforts by someone abusing the public