I tend to do that myself, on the rare occasion I am on a phone call for casual chat, sometimes I pace and sometimes lay in bed. USUALLY when laying in bed it is because I am talking to someone "exhausting"
@Punches his mom's annoying. I think he does it when he has to remember and tells his mom the details of his life and what he's done the whole week. Just seems he's hiding something from me because he has to walk the whole yard too.
When people walk back and forth when talking on the telephone it is to keep a pace in the conversation. To prevent from forgetting what one has to say. Or because what they are talking about they feel very passionately about