短语词组短語IP Centrex 交換機;IP集中用戶交換機Centrex Exchange 中用戶交換機customer centrex 用戶郃用交換機Centrex operation CENTREX 業務VCC Centrex 通信縂機Centrex Compatibility 中用戶交換業務兼容性centrex service 集群業務centrex system centrex 系統CENTREX Centrex 虛擬用戶交換機業務 返回 centrex