Eh, if it's only on pixel phones, it won't be used. We need an open standard that can be used for PCs and phones alike. Telegram already has this feature for desktop/mobile/tablet and it works across OSs. Google and Apple need to catch the hell up.
Mobile networks already have a standard for video over LTE. Unfortunately, nobody seems to have bothered to implement it, at least not on the carrier side.
If you're willing to accept massive amounts of spam when the slightest level of adoption takes place, XMPP and Matrix will also work. Anything that can set up a SIP session, can arrange a video call. Hell, if you have a SIP soft phone running on your device with the port exposed (or forwarded, if you're still on IPv4, but then you also need to set up specific ports for each device), anyone can call you at <account>@<your IP address>, you don't even need a third party if you know those two bits of information!
Getting E2EE enabled will be harder. Not leaking your IP address by simply being called even more so, unless a megacorporation (like Google) or a billionaire with too much money (like Telegram) offer to pay for the bandwidth of their users.