I'M IN THE PHONE BOOTH IT'S THE ONE ACROSS THE HALL

Q. The way to indicate phone and walkie-talkie conversations between two characters seems to be somewhat overlooked in most guides for proper screenplay writing etiquette. Is this just done as a (V.O.) or an (O.C.) when the replying character is not physically present in the scene? I seem to remember reading somewhere an indication of (FILTER) could be used as well. This seems to make the most sense to me, but is it proper?

I usually use (ON PHONE). I mean, why be technical? I also use (ON TV) to indicate that someone is talking on the TV.

All screenwriting conventions are there to make it easier to get the film into the reader's head. I'd prefer to be clear than to be traditional.