Plan organized these local toll centers into geographical groups associated with region within which all toll centers forwarded calls to destinations outside their territory to a Primary Outlet. The Primary Outlet was responsible for establishing an optimal route either to another Primary Outlet or to a Regional Center. Regional Centers were toll-switches responsible for a yet larger geographic region each. The Regional Centers were strategically located across the nation and each maintained cable routes to all other Regional Centers. In addition, they connected to some Primary Outlets in other regions as well, as traffic demanded, or for alternate routing in case of congestion or technical failures. See also Original North American area codes Routing in the PSTN Transmission line References Bell System Public sphere Telecommunications systems Telephony