They don't block the conferencing systems in the regions where they have an ISP monopoly, they block them for all their users because they have a monopoly on the network path to those users. The conferencing system needs to be able to reach 100% of its own customers' users or they'll be bleeding customers faster than the ISP does, so they blink first and pay, after which the ISP's customers have no reason to switch because the conferencing system is working.
https://techcrunch.com/2014/04/28/netflix-inks-peering-deal-...
But another issue is that if it isn't banned, it can happen in private. The ISP doesn't need to actually throttle the target's traffic if it can credibly threaten to and force them into a contract with an NDA, so we have no way to know how widespread it is.