Session Initiation Protocol, better known as SIP, or as some say, "Still Isn't Perfect". With that said, it’s true that nothing is perfect, but you should have SIP in your plans for real-time communications.
Why is SIP needed? In short, it is what allows for interoperability between different vendors’ real-time communication systems and is the common protocol that allows transmission of real-time communications like voice, IM, presence, video etc. SIP runs across private data networks, can be secured to run across the Internet, and can scale up or down dynamically. Why is that important? Because it reduces costs and/or the need to have a dedicated T1 between two systems. With SIP, I can speak to any other SIP system and/or endpoint "dynamically." As long as I am allowed to connect to other SIP systems, and know the SIP address, I can send an "INVITE" to communicate with that SIP end point. Think of it in terms of when you email someone. If you know their email address, you can communicate with them. If you know the SIP address, you can request to communicate with the other SIP endpoint.
SIP also allows for enterprises to quickly consume hosted applications like smart voice messaging, allowing them to reduce the administration time and upkeep of a legacy voicemail system. SIP also allows adding highly productive applications like speech to text, which also makes it convenient for the caller. A service like this allows for new features as they come online. For example, now that you have the audio in text, you can begin to analyze the transcribed audio. This is powerful since you can begin to understand the content of the messages that are being left though the analysis of the text. Another feature that a service like this can enable is gender or emotion recognition from an analysis of the audio. This is really important because in most texting, there really is no way to convey emotion. With newer text analytics engines, emotion in the audio is analyzed and flagged in the text output for a more accurate communication experience.
You will notice that until now, I never used the word PBX or QSIG; instead I used "real-time communications" and "SIP". PBX is purpose built and that is for Voice; QSIG is purpose built for interoperating between PBX’s. SIP is dynamic and is more than just voice. Taking advantage of SIP will open your enterprise to many more possibilities and keep you on the front of real-time communication strategies.