That could be very useful when using a text-based web browser (e.g. TUI)

I don’t know about OP, but it is available in https://thecapslock.gitlab.io/fdroid-patched-apps/fdroid/repo and https://calyxos.gitlab.io/calyx-fdroid-repo/fdroid/repo

There is a payment option inside the app. It uses Stripe to process the payment.

Matrix is in the process of being adopted as the MIMI standard by the IETF. At least it was one or two months ago.

There is also Element Call at call.element.io. I have not tried screen-sharing but it’s worth a look.