As someone with this constant predicament in life, I can only side with the phone girl. I too am guilty of shopping and talking on my phone. I too have carried my conversation on at the checkout. I never ignore the checkout worker entirely; I smile, I listen, I answer their pointless questions about whether I would like a bag. (I've NEVER understood why they ask that: of course I would like a bag – I've got 14 separate, fiddly items from lemons to milk and am only carrying my purse; what do you expect me to do, juggle?) The point is I can interact with the checkout person at the same time as talk to whoever it is on the phone.