What makes SIP so popular is not only that it's deep and flexible, but also because it was purpose-built to engage in multimedia (meaning not just audio but also video and even text) communications over TCP/IP networks. For VoIP calls, SIP can set up calls using a number of IP-related protocols, including the Stream Control Transmission Protocol (SCTP), the Transmission Control Protocol (TCP), and the User Datagram Protocol (UDP), among others. 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 a simple 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 of business-grade PBXes.
In the case of a private VoIP system, the primary telephony system itself is located within the private infrastructure of the end user organisation. Usually, the system will be deployed on-premises at a site within the direct control of the organisation. This can provide numerous benefits in terms of QoS control (see below), cost scalability, and ensuring privacy and security of communications traffic. However, the responsibility for ensuring that the VoIP system remains performant and resilient is predominantly vested in the end user organisation. This is not the case with a Hosted VoIP solution.
To help businesses work from home, RingCentral provides unlimited internet faxing and audioconferencing on all the plans listed above. Video meetings can include up to 100 participants, and meetings can last up to 24 hours—just in case your group needs to burn the midnight oil. Standard plans and higher also include popular communication integrations like 365, G Suite, and Slack.
It is much cheaper than a regular landline - most providers offer unlimited calling plans (including long distance) in the USA and Canada for under $10 per month. There are also some great promotional deals available such as 2 year specials that cost around $6 per month (includes all your calling). Comparing this to a regular PSTN (landline) phone service that can cost in the region of $60 per month can result in savings of over $500 per year for many home phone users. If you also regularly call International numbers you can save even more, with rates typically starting from around 1 to 2 cents per minute to many countries.