how to say goodbye?

Help me please. How to say goodbye when talking on the phone or when writing "goodbye" in a letter?

안녕히 가세요.    안녕히 계세요.

You and  A are here.
1) A leaves and you stay.   안녕히 가세요.         가 is from '가다=go'

2) Both A and you leave  안녕히 가세요.      

3) A stay and you leave. 안녕히 계세요.                

4) On the phone or in a letter.
    If choose one of them, it'd be 안녕히 계세요.   (잘 있어)
    Assume you meet someone on the line or in a letter and you leave first. 
    But it can be anything such as 'thank you. 고마워요.(고마워)' ' See you tomorrow 내일 봐요(내일 봐).'