SIP and trunking to the POTS have been around for more than a decade now. The million dollar question nobody seems to be able to answer is why all our mobiles still use a number instead of addressing via DNS (e.g. user@domain). The carriers wouldn't be cut out of the loop by this, as they can remain a dumb, albeit wireless, pipe. Indeed, some carriers actually implement their systems as glorified SIP trunks on the backend.
Address book software is also plenty capable of tracking email-style addresses. Android has natively supported SIP calling such addresses for more than a decade. This means that for everyone I know using android it's as simple as setting up openPBX on my server. I could have full control and encrypted video calling tomorrow. The trouble is that everyone's favorite status symbol, the iPhone doesn't support this. Which means I couldn't communicate with half my family and many of my clients, as they're not about to install squat to please me.
It appears the reason for this lack of support is the usual modus operandi from Apple. That is to say, they have their own proprietary standard they'd prefer everyone use instead (but that is shoddy by comparison to standard software). Further complicating matters is that new and popular video conferencing firms like Zoom have also introduced yet more shoddy and incompatible software.
While Zoom can bridge to SIP clients, it costs extra, and they already trunk to the POTS at no cost, further entrenching the phone number. Skype has had a similar model for many years. FaceTime users can provide links to allow non-apple clients to call them, but not the other way around. That said, the fact that there is now an HTTP means of doing FaceTime means reverse-engineering the protocol and building a SIP bridge is but a matter of time. When PBXes are capable of appearing to be apple devices with FaceTime things will finally be "good enough" to ditch the number.
Much of the reason for the success of these non-open packages is because the cost structure is largely hidden from users. The FaceTime ecosystem is "free" past the initial phone purchase, and only the host of zoom calls generally pays for the service. By comparison, users of open software and standards bear recurring costs (and they're already paying a phone bill). Like with the telcos themselves, very few people are willing to pay for a SIP account if it's not bundled with hosting, mail, DAV and everything else.
Competing the telcos down from being vertically integrated multi-service providers to mere ISPs is the real mountain to climb here. The first major shared host to execute on this will be able to tap billions in additional MRR. When and if that day comes, I'd likely ditch the cellphone entirely in favor of superior clients on real computers.