the common practice
Approaching push back time one has VHF 1 selected to Ground and transmitts through the side stick push to talk button. (We always use this procedure to talk to ATC.)
So nobody get confused we use the INT switch on the ACP to talk to the engineer down stairs. This way you can listen with your other ear to ATC and you are using different hands to talk to different people.
There is nothing worse that asking ATC if "all doors are closed and locked?"
Hope that clears it up.