i get a phone call everyday but it just rings once, i call back but is always busy, no one ever leaves a voice message, who is this?

this number started calling me after i filled out an online form for health insurance. i am extremely worried about the insurance thing now. how do I get this number to stop calling?

I got a call from 321-281-5713 at 11:41am on 8-20-2008 and when I picked it up it hanged. On the same day I got another two strange phone calls from 407-299-6703 and 407-299-8272 all after I had a dispute with Enterprise-Rent-A-Car at 11:22 am of 8-20-2008 over the phone.

1-321-281-5713keeps calling me hangs up as soon as I answer. When I call back there is a busy signal.
These are quite worrisome.

for the past two days I got phone calls from the following numbers, 407-299-8272, 407-563-1429, 321-281-5713， 407-299-6703，and 702-966-4135. These phone calls either speak nothing if I answered the phone, or leave no message if the answering  machine answered phone. The worst was the phone call from 702-966-4135. Right after I answered phone call of 702-966-4135, my computer is implanted a terrible virus which disables the space bar of my key board. I am connected to internet through my phone line and I guess that is why the virus  could be sent or activated through phone calls.

If Homeland Security is monitoring these reports, I guess they are better off being aware that the owner of these phone numbers might not be just making noice to people, but also could possibly do some damage through implanting virus etc.!