My guess would be that they are charged if they connect with anything, including voicemail. More than likely these people do not deal with real telephone invoices as do the rest of us. There are so many numbers flying around. This call did originate in San Antonio, TX, and came in today at 9:12 AM. Suspect these folk have their own "Service Management" classes as they now ring only three times then disconnect before voicemail kicks in. This just might be costing them something.
2 things here... I have blocked any and all third party callers from being able to ever create charges on my telephone. If they do then Verizon will be on the hot seat in court. I also have a feature on my phone that allows me to block any call of my choice. So when an unauthorized caller tries to reach us by phone I block their number and don't have to worry about them ever again.
Of course the best part is that I do not answer any calls coming in from anyone whose name and number are not on my authorized contact list. We are loving life without bothersome, liars trying to get their hands in our pockets. Unfortunately there are far to many people out there that want to believe and trust in mankind to be fair and honest. Those are the ones that get screwed and cause these vermin to keep doing what they do. It has to be profitable for them to do so or they would quit.