[supertrope]

The former Net Neutrality regulations provided exceptions for an ISP's own VoIP (phone), IPTV and other managed services. That means the bundled phone service is inherently more reliable than an unaffiliated VoIP service using best effort delivery. As long as they did not anti-competitively block or hobble a competing over the top phone service they were legally fine.

On cellular the effect of priority is much more apparent. The "bundled" phone call service has very high priority and can function normally under network stress/congestion. Over the top Whatsapp or Facetime calls will suffer audio glitches or fail to connect under the same conditions.