Hi, I tried a different approach. When I had the stock firmware installed, I have made a backup of the Itercom app with Swift Backup. Then after I have installed Havoc, I tried to restore the app, but I got this message about missing system.apk code. I will try to backup system.apk too, when I have stock firmware again (which I plan to install on sunday).
So I failed too, but maybe it will give you some hint where to look.
The Toolbox app works fine this way.
P.S. Have you tried to fix the in-call volume control with alternative OSes on this phone?