Its most likely advertisments calling from like india, or china or something.
When i was using prepaid I would get these kind of calls every few days and that pissed me off, so I stopped picking up those kind of numbers.

Now I've been using a plan for some time now and haven't gotten these kind of phone calls, only wrong number calls. It would be safer this way because the cost per minute costs less than prepaid. If your cellphone provider offers unlimited incoming calls, then you dont have to worry about it
But if you don't use your phone as often as now, just stick with prepaid and just dont pick up those kind of unknown phone calls.

Hope this helps.

so i had a pre-pay verison phone. i came home and went to use it and all the minutes were gone! so i went online and purchased some more time. while setting at my computer the phone rings so i pick it up but nobody's there.
and all my minutes are gone again! apparently it is possible to "hack" phones and steal minutes? this ever happen to anyone else?

would it be safer to go on a phone plan?
thanks for any help.