Re: 60 calls a person
The way spam call centres work is that they have a warehouse full of people in cubicles with a phone and a script.
At one end is an automatic dialling machine. This dials numbers, and every time someone picks up the phone it allocates the call to the next free spammer in their cubicle, and continues calling.
Now the spammer may be having a smoke, or just bored, and may not pick the call up. So, after a pause of 10secs or so the line is dropped.
That's usually the reason for most voice-free calls....