VOIP command line client / Raspberry PI solution

Hey!
I want to create a "Phone" using a raspberry pi and some buttons. I was looking for a way to get the button input into some sort of client but had no luck finding a good solution. I was looking for command line tools that I could use in a python script but all I found were deprecated or undocumented :(

Since I'm not a VOIP Pro, I wanted to ask here. Maybe someone has a nice tip or solution?! As I said the goal is not necessarily to find a command line tool, it is to get the button input from the PI's GPIOs into a Software that can process audio only calls. I am fairly experienced in scripting, but I don't know how much effort it would be to write a tool from scratch

Thanks already if someone has some kind of Information for me <3