A lot of VOIP is local. So a phone line carries the signal to the office building, and a modem converts it to be emailed or whatever. At least in secure places like in healthcare or finance. On the consumer side, VOIP that you get from say a cable company, also doesn’t travel over the internet. It travels on the same local lines to the cable company, but from there it takes a different route. True the middle might still be digital, but it’s not using internet infrastructure. That would be a waste because there’s no need to be able to send that signal to any given device on the internet. There are a lot fewer landline phone numbers than internet connected devices.