Remove one line from current 5 line plan

I’m in a 5 line plan now. Since I will leave the U.S. very soon, I no longer need this phone number. So I want to remove my line out.

I can’t operate online directly through our plan account (there is only add line option). Also, I can’t find a way to get customer support from real person. The contact phone number (611) is just predefined recording. The online chat is just a robot repeating totally same things. How can I get customer support that’s really helpful???????