which film starts with him in the cinema mouthing the words to the film he's watching? also in the film one of the characters keeps telling people his mobile number ...

Its Play It Again Sam. As it's in 1972 (before mobile phones were invented) Tony Roberts has to leave the phone number of every address he will be visiting so his office can contact him. One really funny line is when Diane Keaton (who plays his wife in the film) says to him, ''Do u wanna tell them the number of the payphone down the street? U might be passing it later.''