I just posted this answer for another problem, seems perfect to help you as well:
There is a nice tool called Autohotkey and I have been playing around with it a little bit and created a fuction similar to your request.
You can highlight a number with e.g. 123/456, then press Ctrl + "-" in the Numpad and the script will copy the number to the clipboard, remove everything but numbers, paste it to zoiper and press the green button.
I put the source code of the custom script at:
There are some more features and I was hoping to get some inspiration or ideas from the Community Forum so maybe there are some people who are interested in driving this further.
Edit: 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.