Technically you could call it a V.O. if/when you reproduce or record their voice in post, but usually one wouldn’t write anything about that process in a script except maybe (filtered) (through phone) or a capped manufactured sound if one wanted to (Or cap a sound that is itself O.S.), so to me… (and I agree to disagree or see it another way, and respect all considerations of other ways and uses no problem) …on the page of script they are under the same slug, exchanging dialogue in the same written scene, so if we are not changing locations visually (INTERCUT) and they are not a narration (because it’s a phone convo), and we don’t see them, but we do hear them, then what is left? O.S.
I like the practicality of the INTERCUT use Charles posted if seeing some of both advances the story.
-Thanks-