After a telephone switching system user has dialed sufficient digits to identify a call as extending beyond the system, the system attempts to route the call over trunks which will result in the least expense to the user. If all of these trunks are busy, the system then searches to find an idle trunk...http://www.google.de/patents/US4486626?utm_source=gb-gplus-sharePatent US4486626 - Method of and system for limiting access to a group of telephone trunks
