Make it possible to switch from phone to video call

I've just played around with the talk app and tested different call scenarios. If I e.g. initiate a phone call from the desktop to a mobile phone I had to decide how to pickup the call. If I touch the phone call button, a normal call is initiate as expected.
On the desktop I can transparently switch to a video call, because the camera symbol is still shown at the bottom of the screen, but on Android I don't have this ability. I have to leave the call and return to the chat dialog and touch the video symbol on top of the window.

Therefore it would be nice if the following function would be added to the app:

1. It should be signaled to the called person, if the desktop caller has enabled his camera, because an enabled video isn't automatically shown on the mobile phone. But if the called person on his mobile device enables the video, this is instantly shown on the desktop.

2. It should be possible to switch transparently from a phone to a video call. For this an additional
   button should be added to the call screen. Currently only three buttons are shown: mute, loud speaker, hang-up call

Environment

Nextcloud version: 18.0.0
Talk app version: 8.0.0
Coturn server set-up and used.

Start by reproducing the phone-call flow described for the Android app using the reported Nextcloud Talk 8.0.0 environment, then inspect the call screen and its transition back to the chat dialog. Done means the called person is informed when video is enabled and can switch from an audio call to a video call directly from the call screen.