The role of an IP PBX is both to insure calls switching within the company, free internal calls, route incoming calls towards the correct recipient with the least energy from your reception desk, and have smart rules configured to route external calls over the cheapest media (Internet, PSTN, ISDN).
Contrarly to IP Centrex solutions, your internal calls do not use your Internet access bandwidth, and confidentiality is preserved by not exiting your company network.
You retain full control over the management of the Call Center (messages, smart routing, calls transfer in case of unavailability, ...).
Focus on the Linksys SPA 9000
Focus son the Linksys SPA 400