Screenwriting : Format for Speakerphone? by Bradley Spence

Hello, all! I have a question regarding how to format a conversation when one is using a speakerphone, or more precisely a smartphone via the hands-free speaker. I have an idea of a scene with one character calling my protagonist, who answers the call on speaker. Of course, someone else is present, and hears the entire gist of the conversation before the protag can turn the speaker off. Hilarity ensues... Does it seem like I have the proper understanding of these devices? ~I don't really think INTERCUT is correct, because I'm not really switching between the two places. ~I don't think it should be (V.O.) because that's usually used for narration. ~I don't think (O.S.) works because that's used when the speaker is at the location, just not viewable on the screen. ~I'm leaning towards doing something like CALLER (via speakerphone), but I have no idea if that's considered "correct". This is my first screenplay, and I've been reading about the craft constantly. But I haven't run across the use of a speakerphone in any texts, nor screenplays I've read. I definitely want to honor the craft by presenting my story like a professional, even though I'm not yet. Any guidance will be GREATLY appreciated! Thanks, Brad