When travelling around, something I have noticed more and more frequently is that people having mobile phone conversations do not behave like other people do. This appears to be especially true when the old-as-time situation of meeting someone walking in the other direction and having to decide which side to favour so you both can pass.
My experience is that people on phones will favour the same way as you, mirrored somehow, which causes a replay and reassessment as their attention is dragged back from the other side of the phone call.
If your body language is there you aren't here.
It's something about projection, I think. Perhaps this is something to do with how we relate to the person we are calling? Like a mirror has been placed in front of us - we relate our own self-image to the voice on the line.