Customers that have groups of stations served from multiple digital switching systems, which frequently include mixtures of PBXs and Centrex, require capabilities that will allow those station groups to operate as one global group. This invention permits customer station groups, including PBXs and Centrexes...http://www.google.es/patents/US5422943?utm_source=gb-gplus-sharePatente US5422943 - Private branch exchange networks
