If you are on Windows you could use Autohotkey, I stumbled over this software and did some zoipering, Hotkeys work even if Zoiper is not active (though it has to be startet). I would appreciate any suggestions or support.
I added the following Hotkeys for the Numpad
Ctrl + Enter = pickup
Ctrl + 0 = hangup
Ctrl + [1-9] = transfer to (*3[1-9]#)
Ctrl + , = take back transfer
Alt + [1-9] = direct call [1-9]
Ctrl + - = strip and call highlighted number
Alt + 0 = show functions
Edit: missed the Link:
I work with the Zoiper Phone Interface 2, as the pressing of buttons is based on the position there needs to be other "coordinates" for other styles.