If the customer really persists and won't budge, I've found just simply asking the customer if they are absolutely 100% certain that the vehicle needs what they are asking for, and nothing else and nothing more. I always follow up with "but I'd really like to just take a look at it for free. I mean, what if it's just a $5 fuse!"
I feel most phone price shoppers just don't know what questions to ask, other than price. I ALWAYS make sure to mention our warranty, loaner cars, ASE certifications, digital multi-point inspections, online reviews, etc.