I can't stand it when a customer comes up to my counter and is talking on the phone, especially when they're using a Bluetooth headset and I have no idea at first if they're talking to me or the person on the phone. Other than the few people who just come up to the counter to buy an item or two, most of my interaction with customers involves either taking their order or bringing their order to them. When I first started, more than once I had tried to take an order while the customer was on the phone and details were left out, and when the order was picked up guess who get's blamed for it?
If I come up to a customer at the counter on the phone, I'll let them know I'll wait for them to be off before we engage. There's just too much involved to deal with someone with divided attention...not to mention its just really rude. The same goes for people answering a call mid-transaction just to have a 5 minute conversation about where they are and why they can't talk right now. Last time I checked, that's what voicemail is for.