What would you say to a customer (without getting in trouble, of course) who assumes you are playing on your personal phone on-duty when in reality you are using your work phone for work tasks?

All I know is that I feel genuinely insulted when customers assume this. I take it more personally than other things they do sick as yelling. Yesterday, I had a lady tell me to stop playing on my phone a d to get to work, in a condescending tone. I took my cart in a totally different direction because I was afraid I'd give a response that would have had me fired.