Underground cable conduit and length

Conduit with a pull string is a great ideal...

does not need to be as deep as other utilities will need. 

Should have no sharp bends thus a flexible conduit should be used, especially if expected ATT FIBER service, copper can take more of a bend than fiber. 

https://www.fiberoptics4sale.com/blogs/archive-posts/95047814-101-guidelines-for-fiber-optic-cable-installation

Do make every effort to pull cables from a conduit in as straight an angle as possible. Pulling on an angle can cause damage to the cable.

1. Underground conduits, if newly installed, should be a minimum size of between 1.5″ to 2″. If the run is long, or if you anticipate the possibility of additional future pulls; Then you may want to install a conduit of up to 4″ (or greater).

Whoever is putting in your conduit make sure the follow the rules for joint trench.  I can't think of anything the phone company would pull that require more than an inch conduit, I don't think there's any requirement, but may want the check the NEC as things constantly change. 

What type of conduit is acceptable for an at&t fiber installation 

800’ buried 

The answers are in the topic. This is a user forum so check with your local utility inspector to see if there's any specifications for your area. 800' I'd do as was stated by @my thoughts and use a flexible conduit probably have to buy a 1000' roll, I don't know if they would sell it by the foot or not.  

@carl4 

800 feet is a long distance, about 1.25 standard city blocks (660 ft per block)

I would recommend splice cases every 250 to 300 feet resulting in a total of 3 conduits. The longest premade cables are 500 feet, providing the tech is even carrying any of these length.

Is there a specific flexible conduit for burial?