KCalc is the default calculator for the KDE desktop environment. Even though in itself not mobile-friendly, you can apply some quirks to make it somewhat usable.
Scaling in Phosh
Use Phosh's scaling command so that KCalc fits on your mobile screen. Be aware that KCalc's App-ID is
$ scale-to-fit org.kde.kcalc true
Go to→ . In the section you may tweak your button & display font sizes.