Welcome to OC.
Dropped their phone!?! My goodness, must be a jumpy bunch.
Just engage them in pleasant, light conversation. Thingswill improve.
For example, ask the cashier something off the usual line of business. "Looks busy, tonight. How do you guys stand it?" Just be careful not to ask something that a jumpy, nervous type might misconstrue as a prelude to a robbery.
To the phone-dropper I might say with a big smile, "You dropped your phone. Careful. I don't want my food prices going up because the staff is hard on the equipment."
A waitress recently said, "That's scary" when I replied to her question that it was a real gun. She was observeably a little nervous. So I told her with a smile, "I'm real tame. Just ask the other staff."