Every phone call you make with just the mic near your mouth and the speaker near your ear, you are whispering into the ear of the person you are calling.
If the person who answers with the mic near their mouth and the speaker near their ear is doing the same thing. It’s like having a conversational 69 where both of you are placing your mouths next to each other’s ears.
It sounds sexual and sensual if you are talking to someone you might be interested in … but it gets a bit awkward if you imagine doing this with a random stranger you would never usually want to get close to.
Gotta keep those waves away from their delicate grey matter.
That or their phone has shitty palm detection and will unwittingly treat their face as a touch if they hold it too close.