I have Kall8 for one toll free number and MCI for the rest of my numbers.  For months calls with nobody on the line have been calling on only the Kall8 number at all times of the day and night.  Finally I logged into my Kall8 account and realized it was coming in on my toll free number at 7 cents a call.  While the amount of $ was not a big deal, the calls at all hours is.  Running a 24 hours emergency service company means all calls need to be answered.
I found Kall8 offers call blocking to area codes that I do not service.  Stopped most of the calls by doing that!  A few new area codes had to be added as some calls continues, and one that I am leaving available (New York City) showing up with calls.  All are from numbers not in service.  I honestly believe it is a mechnical device meant to drive up call volume just for all those 7 cent phone calls and if you leave the phone call connected, it continues to bill.
I have had four MCI toll free numbers for years and never had a problem.  Few wrong numbers, but from real callers.
Filed my FCC complaint, and if I can investigate Kall8 any further to see if they are behind it, I will.