The use isn't simply incidental; it's restricted. You have some piece of equipment which runs telephony firmware that is configured to talk to a particular server, and identify itself with some credentials that bind it to a particular line.
The user is not running a freely selected Internet-based VoIP application connecting to a server of their choice located anywhere in the world. (In which case that would be real Internet use and not incidental.)
The user is not running a freely selected Internet-based VoIP application connecting to a server of their choice located anywhere in the world. (In which case that would be real Internet use and not incidental.)