"Sorry" is a way to apologize for a small mistake you made. When you make a larger mistake and want to apologize more strongly, you can say "I'm sorry for ___ing":

I'm sorry for calling you by mistake.

However, in most situations this would be much too strong of an apology. So just saying "Sorry" is sometimes more appropriate:

Sorry I didn't call you back sooner.

This phrase appears in these lessons: