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A private branch exchange (PBX) is a telephone exchange for a business. A PBX system connects incoming telephone lines to a set of extensions inside a business. Basically, it is a phone system with extensions and auto attendant.

VOIP PBX or IP is a PBX system that can handle calls on an IP network. It supports outbound connection to a SIP trunk or regular trunk; it also supports IP extensions. Most VOIP services provide regular telephone numbers to allow calls from landline or cell phones. The person who calls these numbers usually cannot tell the difference whether the call is make to a VOIP service or traditional PSTN service.

Unified communications (UC), is the integration of real-time enterprise communication services such as instant messaging (chat), presence information, voice, audio, web & video conferencing, fixed-mobile convergence (FMC), desktop sharing, call control, and more with non-real-time communication services such as voicemail, e-mail, and SMS. UC allows your employees to send a message on one interface and receive the same communication on another interface, for example: When you receive a voicemail message, you can choose to access it through e-mail, a visual voicemail application or phone

 Yes. The UC Desktop Client (with voice) provides all the PBX calling functions with the builtin softphone client. In addition, it offers UC features such as Instant Messaging, Visual Voicemail, ClickCall, etc. from the same client.