The answer is, "Yes," Microsoft does offer a PBX solution called Lync Server. As to how Microsoft can replace a PBX, this can be a hot button topic. Because Microsoft is a relative newcomer to this technology, in some people's eyes, Lync Server is not ready to be a traditional PBX replacement.
The earliest forms of PBX used by organizations were physical switchboards and required operators to patch calls manually. Thank goodness for progress. While there are a number of different types of PBXs with a variety of different acronyms, PBX is often loosely used to refer to any number of switching systems from manual (PMBX), to automatic (PABX), to software-based (IP-PBX).
The essence of a PBX is that it can establish connections, maintain connections, provide call information, and disconnect sessions between callers inside and outside of an organization. In a modern PBX, it is expected that there will be additional functionality provided, such as an auto attendant, call forwarding, call transfer, conference calls and voicemail.
Does Microsoft's Lync Server meet the criteria of a PBX? Absolutely. Like many current IP-PBX platforms, Lync Server uses software-based switching to provide Voice over IP (VoIP).Lync Server uses TCP/IP networks and protocols like SIP, RTP/SRTP (RTaudio / G.711 / G.722), and RTCP to perform its PBX functions. For internal users with Lync clients, Lync-qualified or Lync-compatible devices, it provides both peer-to-peer calling and conferencing. For external communications, the enterprise voice functionality must be configured and the mediation server role must be deployed. With the mediation server role deployed, Lync Server can also connect directly to an Internet telephony server provider (ITSP) via a direct SIP trunk, or it can connect to a qualified or supported IP-PBX or gateway from another vendor in order to communicate with callers on other PBXs or the public switched telephone network (PSTN).
Does Lync have feature parity with your current PBX? That may be the ultimate question for you and your organization. In general, if Lync Server can provide you with the equivalent or more features than your current PBX, and the price is right for your budget, the answer may be straightforward. To meet the needs of enterprises as a PBX replacement, Microsoft has continually improved voice features from Office Communications Server 2007 to Lync Server 2010 -- which replaced OCS -- and Lync Server 2013.
Lync Server can be integrated with a wide variety of voice platforms. If you like Lync Server, but need to maintain portions of your existing voice infrastructure, you can configure some level of integration. Even if the current PBX or IP-PBX is not on the list of qualified or supported IP-PBXs, it is possible to deploy a qualified gateway that will allow you to do the signaling and media transcoding necessary to interoperate.
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