Last time I was in GF I did ask a walker and talker to 'moderate his volume' and to be fare he did and then came over apologised once his call was finished.

He did say he didn't want to disturb the people he was with so I responded 'so it's ok to disturb everyone else?' to which he looked a little shame faced.

I do find the finger to the lips in the 'shhhh' position works in most cases.