Taking a poll here: What's your preference for a character who is not visible, but heard - speaking on the phone (via speakerphone for ex.): (V.O.) (O.S.) or (O.C.)?
I'm with you Bobby - VO just doesn't feel right. Ultimately, I think you go with whatever works best for you and adjust later if you get comments about the story being interrupted because of how you portray the phone calls . . . good luck!
I've used varying styles. Mostly, however, I utilize (ON PHONE) as an extension or (filtered, phone) as a parenthetical. It's all a style choice. Whatever works best in the scene and fits with the overall tone.
