The original pole barns were indeed built with round poles.
But they were pretty breezy.
As people wanted them to be more weather proof the need for better joints made the switch to squared poles easier than cutting flats on round poles.
If you are just after a shed to keep stuff out of the weather round poles are fine.
If you are looking for an alternative to a tight, stick built building, squared off poles will make the carpenter work easier.
Around here there is a big difference between “telephone poles” and “power poles.”
Telephone poles are shorter and smaller. 8 to 12 inch dia and around 20 to 25 feet when cut off at ground level.
Power poles are 15 to 24 inch dia and 40 to 60 feet long when cut off.
Remember, you are going down at least 4 feet into the ground when shopping.
This will get you up to date
http://www.easybuildings.com/