I don't think it's the call itself that's annoying, that's just a smart thing to do, it's the customer being an ass towards the employee that's the issue. Like when a customer asks to double check the back, they can ask politely & reasonably or they can be a complete ass about it and act like we're squirreling away half the store out of sight.

In this case it's the customer acting like the online inventory is the be-all-and-end-all arbitrator of what the store has in stock and then belittling the employee (who has 0 control over the website, stock, or pretty much anything) for being "wrong"