Centrex is a service provider-hosted PBX service that typically offers a basic set of voice features for end customers, primarily for small and medium-sized enterprises that are looking for an outsourced, hosted communications solution. Centrex delivers such services as call hold, call transfer, last number look-up and redial, call forward, three-way calling, but does it on a packet-based network. Latest centrex systems employ IP telephony call control and feature applications and are owned, hosted, and managed by the service provider in its own network, and that capacity is shared across multiple customers.

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