I know precisely nothing about entrepreneurship or starting new projects, so I'm probably not the best one to ask having never done anything real on my own.
Zoom/Meet/Teams feels like work and people usually do the calls on a computer. I want this to feel more informal and on the phone just like when someone calls you on whatsapp. The idea is that everyone on the call that you set up would receive a request to join at the same time.
I don't feel that way at all. Those are all just apps. Remote meetings use them, but that doesn't mean that corporations own the usage of those apps. Frankly, your idea of everyone getting the invite at the same time feels more like a work-related need than a personal need. At work, we want efficiency - everyone shows up, talks, then signs off. For a personal call, being able to come early and stay late sounds far more friendly.
At the end of the day, this is all up to you. If you think there is a need, there are probably others who agree with you. But before you build, ask around to find out how many there are, and if they agree enough to pay money for the things you would build. If not, this will be a weekend project, not a business. (Which is also fine, BTW.)
Could you build it as a friendly wrapper around Zoom/Teams/Whereby.com/BigBlueButton/Teams/Skype etc. to save yourself work?
As a value add, it would be nice to have interoperability between all video calling apps (that's clearly a separate, much harder to build, app), because agreeing on an app is tedious and wastes time.