Cool, thanks for the lead, I found this ”Due to the length of the cable routes and the inevitable leaks along it, the pressure must be re-established in the system at various points or the cable will be unprotected at substantial distances from the central office. One popular way of raising air pressure in the system is by using air pipe that follows the cable route and introduces pressure at various fixed points. Air pipe manifolds are connected to the air pipe to distribute air to the cables in the designated utility holes." http://airtalk.com/newsletter5.html telephone lines are much more complex than I imagined.