I live in the balkans... if people call someone, they call someone using a phone. If they use any of the internet-call services (from facebook video, to skype, viber calls, whatsapp, duo, etc...), they do it because they either want video with their calls or because the normal calls are expensive (for example, a minute of calling from slovenia (EU) to serbia (non-EU) costs 0.59eur, same for eg. switzerland... calling from serbia/switzerland back home (while roaming) is even worse - 2.65eur/minute).

So, to call people via an app, those people need an app installed (and an account on that app). Since most people here have androids, they don't have duo preinstalled, but most of them use either Viber, facebook (which now serves ads) or whatsapp (which is technically facebook, so ads could come there too).

Yes, i have no problems with setting up a sip server, sharing the credentials over irc and then using wireguard on one side and ipsec on the other to protect the sip session. My mom only knows how to use whatsapp, because that's the only thing she ever needed, because everyone she calls has whatsapp installed. Explaining how to install duo, and then calling my aunts and uncles and telling them why they need duo is a pain in the ass, especially if they're already happy with whatsapp. I have no idea how many old people you know and have dealt with regarding technical stuff, but if you don't believe me, you can try migrating my family to signal... and good luck with that :D