I started working at a tire shop years ago, and the manager told me to never turn away from a customer in front of me to answer the phone.
I agree 100%. The guy in front of you is there now, and if you treat him right, he's there to buy. The guy on the phone is calling everybody in the book and trying to find someone $0.05 cheaper than the last guy.
I pulled wrenches on a large farm (over 100 pieces of equipment, from freightliners and macks, to pickups, combines, swathers, and tractors as big as a Versatile 1150. (that's an 1150 cubic inch turbo diesel...mmmmmmmm) and was talking about that with the manager after work one day over a beer. He shared my view of a counter guy treating you like dirt to keep answering the phone.
He said, "Next time, just turn around and walk away." It was a fairly small town, and a 56,000 acre farm bought a LOT of parts.
Next morning I was at our usual truck parts place waiting to get a part we ordered in. When the guy reached for the phone to answer the 3rd call, I said, "Could someone else get that?" He ignored me and took it anyway...while he was looking up an air fitting or something for the guy on the phone, I left and went to the truck parts store across the highway. The part I left behind on a pallet was a complete 3rd member for an International floater truck.
By the time I picked up everything else and got out to the farm, they had already called the manager. He was a bit shocked, but told them "I'd have done the same thing...bye."
We had a good laugh when I got there.
The truck wasn't needed for a couple months when we were to start spraying, so ordering another one wasn't a big deal.
If the phone are that important, have counter men, and phone men.
As far as parts go (and I owned 2 jeeps so I consider myself an expert..haha) someone's not going to drive from store to store hunting for a best price and wasting your time. If they made the trip, they're there to buy something. Look after them.