I use basic etiquette, if a customer approaches me and they are talking
on phone or headset I state quite bluntly
"When you have finished with your current conversation I will talk to you"

followed by "Now how may I help" but thats me :)

Personally I prefer Bluetooth as the transmission is low powered and range is down to a few meters
so no cell phone wavelenths next to my head

If I meet an uptight customer I tell them they are being rude
and will not be served at which point I promptly tell my
manager what has happened
either way it is situation dependant
i.e. your manager, the customer and yourself

If I get a very rude customer I just use blunt truth
with no emotion, works for me.