Can "Call Forward - Busy, don't answer" feature be added to the system?

it is already on the system. Relatively new feature. Look into "Call waiting", You want to disable call waiting, so that if somebody calls you while on a call, they get forwarded to voicemail or something like that.
It's in the admin portal > Users > click on a User > Phone > search here for call waiting.
It's also possible to configure it through the settings on the app.
In the desktop/web app, go to Settings > Phone > Call waiting