Functions that can't be CALLed

REM, OPTION, and XON/XOFF are instructions that aren't recognized by CALL. Are there others?

Those aren't functions though OPTION is a compiler option and XON/XOFF are sort of keywords I guess REM is the same as ' You can prove this by doing:

DEF OPTION
END

DEF XON
END

DEF XOFF
END

There are also the console commands LIST, CLEAR, NEW, RUN, and CONT, the constant PI(), and other keywords like GOTO etc.

Basically, any instruction that doesn't use some kind of typed parameter can't be called.