i want to know which you should say, which is the proper way of telling someone that you will always be there whenever they call.
Here is how I see it
“To call on” means something along the lines of “to summon” or “to bring forth”. I don’t think this is what you would want.
“Call for me” is probably better. It means something like “call me whenever you can/need”
Of course, the best thing would probably be to just say “Call me”. That covers everything, I believe.
I also apologize that it is kind of difficult to explain what I mean 
I hope you understood!
Hi,
Probably the commonest expression would be: Call me any time.
Alan