TBH I think it is wrong to have serving staff answer phones at all if it can possibly be avoided. But if that was the way the shop was set up, I would expect the person to answer the phone and say, "I am serving a customer at the moment, would you like to hold for 10 minutes or would you prefer to call back?" Or similar.
I would decide at the time whether or not I considered the situation reasonable. The criteria is not really whether the call is or is not business-related, but whether or not it is *important* enough to keep a customer waiting. I'm tolerant of personal calls when they are non-trivial, urgent and as brief as possible (arranging an emergency baby-sitter, instructing a car mechanic etc.)