KCalc

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.

Simple calculator layout in Portrait with both font sizes set to 32

Scaling in Phosh

Use Phosh's scaling command so that KCalc fits on your mobile screen. Be aware that KCalc's App-ID is org.kde.kcalc:

$ scale-to-fit org.kde.kcalc true

Larger Buttons

Go to Settings→Configure KCalc.... In the Font section you may tweak your button & display font sizes.

Numeral calculator layout in Landscape with button font size set to 31