18,000 feet is over 3 miles. I doubt your buildings are that far apart. What you could do to help insurnace sucess is: Check both the pairs you are going to use for the demarc extension by grounding the Tips and Rings at one end and with a VOM check tip to ground and then ring to ground on both pairs. You are looking for resistance within a couple ohms of each other. Then put a Tip to Ring loop on both pairs and check loop resistance again you your looking for two pairs within a couple ohms of each other.

Forty six years and still fascinated with Telecommunications!