More on Phone Dialog

Q. Do you have to write into phone) after every line of dialogue into a phone?

JOHN
(into phone)
I'm talking into the phone.

A professor of mine absolutely requires that I do that.

Well, I don't like to do that. However a showrunner friend of mine insists that it keeps things clear. So I actually do it even though I don't like to.

However, I put (INTO PHONE) on the character line so it doesn't take up space:

JOHN (INTO PHONE)
I'm talking into the phone.

If we hear the offscreen person's dialog, then her character name would be followed by (ON PHONE) or, old school, (FILTER):

JANE (ON PHONE)
Like, duh, yo.