A couple of months ago, I forgot to put my phone into "silent mode" before playing my little once-a-month one-hour performance, and it rang while I was on stage (!!?!?).

Fortunately, I was between songs at that moment. I answered, said something along the lines of "I'm busy right now" ~ audibly, near the microphone ~ and tried to turn it into a little joke for the audience's benefit. I told them I was grateful that they had turned off their phones, and was sorry that I had forgotten to do the same.

Back to the incident reported int he opening post, having the phone ring at an inappropriate moment is a forgivable oversight, but answering at length and carrying on a loud conversation is quite another matter which deserves a good scolding (at least).