I also work in a C-Store - I hate it when a customer comes in gabbing on the phone - throws a 20 on the counter and says pump 5, in between sentences of his phone conversation.
I feel your pain. I work at the front desk of a hotel and half of the time, no wait, make that most of the time, when a guest comes to the counter while on their phone, I have no idea if they're talking to me or the person on the other end of the "line." No manners whatsoever.
I've lost count of the number of times I've replied to something I thought was being said to me but turned out to be said to the person they were talking to.