-) Allow clipboard paste of number in whole app (not only in 'invisible' number text field)
Maybe if Ctrl+V - activate call display and paste the number...

-) Add a button to clear the number text field (may be an X left of the call button

-) Add a setting for a global shortcut - may be Ctrl+P (or make it configurable) - maybe call the same function as via tel link - expect the phone number?