What Is SIP? Session Initiation Protocol Explained

When you communicate with another person online, you need a reliable technological foundation through which to communicate. Session Initiation Protocol (SIP) is a common VoIP protocol that helps initiate online communications and phone calls over Internet connections. In this article, we'll discuss SIP, how it works, and the benefits of using SIP.

What is SIP?

SIP stands for Session Initiation Protocol. It is a method of VoIP (Voice Over Internet Protocol) that initiates, or enables, messages to be sent between two endpoints over the Internet. Session Initiation Protocol allows for many types of information to be exchanged over the Internet, including voice, video, instant messaging, and even media files.

What is SIP calling?

SIP calling is the process by which voice calls are transmitted via an Internet connection. However, in order to fully understand SIP calling, you should be familiar with each of the components of the acronym: "Session," "Initiation," and "Protocol."
people sitting around table with laptops, ipads, and phones connected

A session simply means the period in which two devices exchange information over the Internet. Think of a session as a video conferencing call or an instant messaging chat.

Initiation refers to the act of starting a session, keeping the session online, and ending the session. It's comparative to pressing the "call" and "end" button. SIP handles the technological processes that take place when you click those buttons.

Protocol is a computing term that essentially means a set of rules that control exchange of information between devices. It is a means to ensure that devices understand each other and can communicate clearly over the Internet.

So, adding all three terms up gives you a protocol that lets you initiate live communication sessions with others over the Internet.

How does SIP calling work?

SIP involves two-way communication whereby one device sends a request and the other device receives the request and responds. It works by coding the requests and running automatic checks upon initiation of the call and throughout the call. It checks that:

  • the recipient maintains an Internet connection
  • the recipient is available to receive a communication
  • the recipient's device is compatible

Session Initiation Protocol then ends the connection between the two parties when either party hangs up. You can purchase dedicated SIP phones for using Session Initiation Protocol communication methods.

What is SIP trunking?

SIP calling is primarily made possible by Session Initiation Protocol trunks. But what is a Session Initiation Protocol trunk?

A Session Initiation Protocol trunk is basically a digital phone line that allows users to make and receive calls from anyone with a phone number, whether digital or landline. Instead of a traditional line that uses a physical PBX (Private Branch Exchange), it uses a cloud-based system to connect your physical phones to the Internet. This is referred to as an IP PBX. PRI (Primary Rate Interface) lines also use PBX communication; however, Session Initiation Protocol is still the better choice, as it offers greater flexibility for the user.

SIP trunking is a huge technological advancement as it allows businesses to replace their traditional phone lines with "virtual" phone lines. It works through the use of a packet switch network where communications ultimately turned into digital packets and sent across a network like anything else that is sent over the Internet.

SIP vs. VoIP

Session Initiation Protocol and Voice Over Internet Protocol are not two separate, competing protocols. Instead, they work in conjunction with each other. Essentially, VoIP is the underpinning technology that allows for voice calling over the Internet. Session Initiation Protocol is a specific piece of that that enables the initiation of calls and maintains the connection throughout.

Additionally, SIP allows calls without a computer where as VoIP calls require a computer.
Hand reaching out to push VOIP button

Benefits of SIP calling

There are many obvious benefits to using SIP calling as opposed to traditional phone lines. Here are a few:

  1. Flexibility. SIP calls, as opposed to traditional calls, ultimately conducted on a wide array of devices, from mobiles devices to tablets and laptops.  This allows employees to access their business' phone lines from anywhere. This is especially important as more and more businesses have moved remote.
  2. Scalability. Because SIP operates on the Internet, it boasts a much higher data capacity than traditional copper telephone lines. This means that calling up operations is as simple as clicking a button as all the data operate on the cloud. Traditional phone lines would require installation of additional copper lines.
  3. Easy to implement. Whether a business has an existing system with desk phones or not, SIP systems are quite easy to implement as they are virtual. Furthermore, implementing a Session Initiation Protocol system involves digitizing an existing physical phone system.
  4. Reliability. Because SIP doesn't rely on built-in hardware, it's much more reliable than traditional phone lines. With traditional phone lines, many things can go wrong, from the PBX to the copper lines themselves. Though with any software, there's always a chance of failure, SIP calling minimizes those chances.
  5. Affordability. With SIP, you have on installation, hardware, and maintenance costs. However, with most Session Initiation Protocol service providers, you only pay for what you use. SIP trunk pricing differs from provider to provider, but all provide cost savings compared to traditional phone lines.


Session Initiation Protocol is certainly a no-brainer when it comes to modernizing any business' phone system. SIP trunk services offer much greater flexibility to allow businesses to communicate and get business done more efficiently in the ever-changing technological landscape.