I personally have never ever had a mobile in use at the check-out, but I have on several occasions had to wait whilst till operators have enjoyed a chat between themselves (and usually not about work - or if about work, nothing to do with me). QED I take the stance that if my mobile is in use whilst I am at a checkout then the checkout operator should mind their own business, not try and join in the conversation, not seek to comment on my conversation, and most certainly not complain about that fact that my mobile was in operation - provided, of course that I (or someone with me) was able to conduct a purchase on my behalf without causing anyone any inconvenience.
The till operator needs to come to terms with the fact that the job is one of providing service and not criticising customers.
Em
