It was never a problem for me on the Post Office counter as mobiles were still quite rare then. But My brother would just sit there and say: "that's ok, go ahead and make your call, I'll wait until you've finished". As he worked in a very busy office and lots of other customers were waiting, it was usually very successful. But I have to say I agree with the cashier on this one.
"Goodness is about what you do. Not what you pray to."