If someone calls me as I'm about to buy something I don't stop talking. All I have to do is hand over the money or card which is not hindered by being on the phone. If the cashier misses out on asking me how my day was I'm sure they'll get over it. I'm not there to talk to the cashier, I'm there to buy something.
I haven't heard a customer yet say "excuse me,---- but I've got to get off the phone to make this purchase." If there was an issue during the transaction( a price discrepancy, coupon issue, etc.) then I would think the customer would just tell the person on the line to hang on a sec.
I just don't get where the idea of "owing" respect or courtesy to the cashier comes from. Certainly one shouldn't go out of the way to be rude, but it's not like a customer goes to a store to chat with a cashier.