Long before the appearance of VoIP, nonswitched alternatives to the PBX have been available. For systems of less than 50 users, Key Telephone Systems (KTS) share outside lines directly and have dedicated intercom lines to talk between stations. Current generation key systems are more PBX-like than ever, so it may be hard to find that distinction anymore. But older key systems won’t support advanced switching features like trunk-to-trunk transfer that can lead to toll fraud. Still, so-called hybrid key systems should be treated like a regular PBX when it comes to security.
Centrex, IP Centrex, and Hosted IP-telephony services are carrier-based PBX alternatives that provide a private dial plan plus the more popular switching features that an on-site PBX system might. However, the switching equipment stays in the carrier’s infrastructure and is managed by the carrier. This is a mixed blessing since it’s likely to reduce the overall functionality and access policy tailoring available to you if your organization uses such a service, but it does mean that the carrier shoulders a larger share of the responsibility for any toll fraud that may result (and consequently won’t provide high-risk services like trunk-totrunk dialing without extra security measures).
More recently, the appearance of IP telephony has provided an opportunity for some manufacturers like Avaya to rearchitect their overall PBX approach and separate the functionalityonce provided in a single device into multiple devices. In particular, call control and signaling can be separated from media processing and gateway services; this approach makes possible an architecture where a few call control servers can provide redundant services across an entire organization with media gateways located in every geographic location that contains their physical presence. Well treat this approach along with other similar VoIP architectures
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