I believe that is possible and you have mainly two options:
The first option is to use AppiumLibrary or AndroidLibrary (Calabash) to do phone calls with Robot Framework in an android phone. It's not an easy option. You would need to discover the locators of the phone application, or the coordinates.
https://github.com/serhatbolsu/robotframework-appiumlibrary
The second option, I believe its more accessible, would be to use a Windows VOIP client and control it using for example the AutoITLibrary.
https://code.google.com/archive/p/robotframework-autoitlibrary/
In fact there is also a 3rd crazy option: you could hack an old phone, and create a Robot Framework custom library in Python that would control the phone with a Raspberry PI and GPIO, connecting the keys switches to the GPIO pins.
That would be a nice hacking project =:)