I was wondering that too. I can't imagine there would actually be phone lines up there. Also it would not survive the weight of all the snow that would pile onto it.
The 547 prefix is u-district if I remember rightly. Might have been an engineering dept. prank. Look around for a fully assembled Volkswagen in a tree....
I'm sure this is simply a part of the homeland security net that has been cast over all border states. If we knew the correct number to dial a secret stairway would be revealed. I know this is true because I saw it on TV in the 60's. Remember Maxwell Smart and Agent 99 (YEOWWWW!)