But it can also handle other functions, including session setup (initiating a call at the target endpoint—the phone you're calling), presence management (giving an indicator of whether a user is "available," "away," etc.), location management (target registration), call monitoring, and more. Despite all that capability, SIP is simple compared to other VoIP protocols primarily because it's text-based and built on an easy request/response model that's similar in many ways to both HTTP and SMTP. Yet, it's still capable of handling the most complex operations performed by business-grade PBXes.
The setup process of a standard small business VoIP phone system is refreshingly simple.

System complexity: Some businesses we speak with tell us they're replacing their existing phone system because it's too complicated and not user-friendly. Buying an overly complex system can be as bad as buying one that has too few features. Apart from paying for features that won't be used, advanced systems can present usability issues.
The more feature-rich plans include team collaboration tools, conference bridging for up to nine people, call recording, and Nextiva Anywhere. Nextiva Anywhere is a feature that allows users to make and receive calls from any device, anywhere—all using one number.