Use of Comma Before Because

Do not put a comma before because UNLESS there is a negative independent clause before. You do not always need to put a comma before because. You have to insert a comma if a negative clause or statement before because may alter the intended meaning or mislead the reader. Example:

I didn't call her because I missed her. I did it to ask something important. 

(It means I called her. However, missing her was not the actual reason)

Now, the meaning will completely change if we put a comma before because:

I didn't call her, because I was mad at her. 

(It means I did not call her at all. Moreover, the reason for that is the one that comes after 'because')