Some time ago I was a Vonage customer. We had a Vonage line for my home office to compliment the POTS line that service the house. Our only internet access was via DSL over that POTS line.
We haven’t had a POTS line here since 2004.
While Vonage was a pioneer in what we now call-over-the-top internet telephony, for most of its existence the companies primary means of delivering service was by way of an “analog telephony adapter” or ATA. An ATA provides the RJ-11 connection required to connect to a traditional telephone.
Service providers using ATAs are essentially emulating the PSTN. It makes perfect sense since they want to offer an easy, drop-in replacement for traditional phone service. The advantage that they sell is simply that they’re cheaper. Most care little for esoterica like HDVoice.