This used to be a topic about other guests in shared vehicles (on which I held and hold strong views) but now it seems to have spread to Driver /Guides to judge from some recent posts.
I am lucky in that I have one or two Safaris under my belt now and often get my 'old' driver who knows my preference without being told.
I am no authority on etiquet but I do not allow any driver to use his Mobile phone whilst 'on duty'. They are switched off! New drivers are told that on Day 1.
There is simply no need for them to do this. It is not as though you are with them 24 hours a day. They are meant to be professionals and clients must make their wishes known.
So what can newcomers do?
Well ask/tell him politely but firmly to desist. If by the second day if he is not complying ask him to ring his boss for a replacement.
You could try the carrot and stick. If the phone goes off tell him not to answer it. If he does, then say "That's $10!", 2nd time " $20!" and explain that this is how much is coming of his tip at the end of the trip if indeed he lasts that long.
His job is to look after you 100% and find and point out wildlife to you whilst telling you asll about it. It is NOT to say "Lion, over there!" and head into the Blackberry stoop. (He can ring his wives, girlfriends, other guides etc., when you are in the Lodge!)
So that is my view and I would welcome the opinion of others.