Unless you're some Gordon Gekko type player finalising a deal with Mr. Yagamoto in Tokyo I don't think it's too much to ask for you to take one minute from your hectic Wall Street schedule and exercise some common courtesy and basic politeness.
again, all these assumptions are based on the idea that customers are there to engage in some kind of discourse with a cashier. If I'm running in to some place to grab a box of cigarettes(I don't smoke) and I throw ten bucks on the counter while I'm talking to someone on my phone, I'd be pretty miffed If I was told that I'd have to get off the phone just to have the cashier hand me change.
You don't have
to do anything. It would be a nice respectful thing to do though.