Recently installed Zoiper5 Pro. Was having issues with the application disappearing when minimized.
Found a solution with the Gnome shell appindicator extension. I first tried installing it via apt, which was unsuccessful. Then I tried the Gnome Web epiphany browser method and installing from here.
That worked and I can now access and minimize Zoiper from the tray.
Zoiper5 does not yet support Wayland btw, so X11 much be chosen as display manager when logging in.