These calls are hackers attempt to use our lines. They are systems that call a series of random numbers probably all users of PBX or IP phones. They make your lines available through their network and their users " probably buyers of call cards" can make their long distance calls, At your expense.
I have no business with any USA company. I wish they would stop calling. I receive calls from USA daily. What a waste of time to deal with them. If I see any area code that I don't recognize, I just pick up the telephone and slam it down.