Session Initiation Protocol: What is it and what do you use it for?

The term "SIP calling" can sound daunting. What does it mean? What do you use it for? Fret not. We're here to break it down just for you.
Picture of Nigel Seah
Nigel Seah

Digital Marketing Executive

Reading time: 6 minutes

So you have a business problem.

And when you finally find the website to the business that provides the solution to your problem, you dial up their company, only to be directed to an Interactive Voice Response (IVR) menu. Yay, you? 🤷‍♂️

A Vonage study revealed that 61% of consumers believed that IVR tech made for a poor customer experience.
You’re not the only one who dislikes IVR call menus.

In fact, a 2019 study by Vonage revealed that 61% of consumers felt that IVR technology made the customer experience poor. If only if there was a way to directly and automatically connect from your website to a call on your office phone. 🤔

Enter Search Initiation Protocols (SIP).

Sounds too technical? Fret not. We will be breaking down what SIP calling means exactly and how it can benefit you. But just to give you a quick teaser, SIP is basically a cloud version of your traditional office phone system, the Private Branch Exchange (PBX) system.

Now, let’s delve into the details!


What is SIP Calling?

Before we go into the nitty-gritty and super techy details, let’s break this term up and explain what it means part by part.

  • Session – A session is basically any form of live communication. This can come in the form of phone calls, or even video conferencing via apps like Skype, Zoom, and Discord.
  • Initiation – noun. the action of beginning something. (so, session initiation means starting a session. Following so far?)
  • (internet) Protocol – It’s a set of rules that defines how two or more computing devices communicate with each other. Such devices include laptops, smartphones, routers, and network switches.

Put together, SIP initiates (and ends) a session between 2 or more people. Each individual is known as a SIP client, who owns a SIP address.

SIP initiates a session by sending data between 2 or more SIP addresses. Examples include a Whatsapp call or a Discord video conferencing.

P.S. SIP is sometimes confused with Voice over Internet Protocol (VoIP). But VoIP is actually an umbrella term that refers to different types of technologies that allow for voice and video communication via internet connection.

How does SIP Calling work?

In trying to understand how SIP calling works, you need to understand 2 things.

Firstly, SIP doesn’t work alone. It works with other protocols to allow a session to happen. Next, you need to get an overview of how a traditional phone system looks like.

Let’s take a look, shall we?

Protocols involved in a session

There are 2 groups of protocols involved in session initiation. The Application Layer and the Transport Protocols.

The Application Layer specifies the communications protocols used by hosts in a session. Meanwhile, the Transport Protocols are the ones that transmit data packets during the session.

Sounds too technical? Don’t worry. Let’s examine them using layman terms.

Application Layer

1. Session Description Protocol (SDP)

The SDP transit data helps recipients join a session.

This information includes the session description, time description (amount of time the session is active for), and media description (details regarding the type of media that will be transmitted during the session).

An SDP may, for example, let the recipient know that the session will be a video call which will take place at 10 am PDT.

2. Real-time Transport Protocol (RTP)

It’s a protocol that is used to stream audio and video in real-time. It works alongside the  RTP Control Protocol.

3. RTP Control Protocol (RTPCP)

This protocol relays information related to the quality of each session.

Think of it as a data analytics tool in many software. It tracks certain metrics to provide administrators with insights as to how well the software is performing. RTPCP does just that.

Some of the information it transmits includes the number of data packets exchanged, the number of data packets lost, as well as the amount of lag time.

Transport Protocols

1. Transmission control protocol (TCP)

TCPs transport data packets in an ordered sequence and in return, a receipt acknowledgment packet is sent back.

If an error occurs along the way, say the acknowledgment packet isn’t received on time, the original data packet is re-sent. TCP is designed for accuracy and ensures data packets are delivered in their original sequence.

2. User datagram protocol (UDP)

UDPs were not designed to transmit data packets in the correct sequence. Rather, its goal is to simply ensure that the data packets are delivered to their destination asap. It does not detect out-of-sequence packets or retransmit lost packets.

SIP’s role in a traditional phone system

A traditional phone system comes in 3 different parts. The PBX, the Primary Rate Interface, as well as the Public Switched Telephone Network (PSTN).

You can think of PBX as your traditional office phone system. It is a system that connects phone lines within your organization or company.

The PRI is actually the group of lines that connects your calls to the PSTN.

And the PSTN is the network which then routes calls to their final destinations.

An SIP calling system replaces the PRI, allowing you to bring your landline onto the cloud network. But why should you do this?

Traditional PRI can only handle a maximum of 23 conversations at any point in the US. But a smaller company does not need to hold that many conversations at once. Also, as companies expand, so to will the number of conversations happening. In other words, a PRI is super inflexible.

Replacing the PRI with SIP is not only more cost-effective, but it also allows for a greater number of possibilities such as remote work.

Benefits of SIP Calling?


As we’ve mentioned, the replacement of traditional PRI with SIP allows for more flexibility in organizations.
Businesses can now have as many conversations simultaneously as they want. No longer will companies be restricted by the fixed number of conversations provided per PRI per country.


Because it is so flexible, it makes communication easy when you want to scale your business.

Because SIP calling relies on an internet connection, companies no longer have to buy local PSTN networks just for the calls to be delivered to the recipients.

With the rise of remote working, it is essential that businesses use telecommunication systems that facilitate international communication. With SIP calling, organizations need not be held back by the fixed number of conversations permitted by PRI. They can easily add more employees into their connection network with ease.


The monthly phone bill can reach insane amounts if a business operates on a traditional phone system. Especially if international calls are made.

With SIP calls’ reliance on the internet, international phone calls are made locally, thus making them a lot cheaper.

Reduces disruption to operations

We are living in unpredictable times. The regulations put in place to curb the spread of the COVID-19 pandemic have forced employees everywhere to work from home.

Despite this disruption to operations, businesses can still run due to the presence of apps that help facilitate communication. This includes video conferencing apps like Skype, Zoom, and Discord.


Sometimes, unforeseen situations happen and your company’s internet may crash.

But with the use of SIP calls, you need not worry about blackouts and system failures. Your service provider will reroute your calls to employees’ mobile devices or other offices.

You can be assured that business-related communication will not be jeopardized during such events.

Better customer experience

As we’ve stated at the beginning of this article, IVR technology is the bane of your caller’s experience.

SIP call eliminates the need for the IVR phone menu and allows your caller to connect directly to your company via your website. And no, I wasn’t talking about them picking up their phone and dialing the company number stated on your website.

I literally meant that your website is used to connect your business to those who wish to call your company.


Novocall Conversations is a call-back widget that uses SIP to connect the caller to your company directly.
A call-back widget makes use of SIP to connect the caller to a company directly.


For example, a call-back widget embedded onto your website allows someone to select a timeslot which he or she wishes to receive a call-back from your company. This allows for a direct connection without the need for an IVR menu.

Who should use SIP Calls?

Well, everyone really.

Those who are still relying on traditional PBX systems to conduct business-related communications would have felt the inconvenience caused by it one way or another.

The fixed number of conversations allowed to happen at any given time makes it more inconvenient for companies who want to employ more people freely or expand the business.

Also, since SIP calling makes international calls cheaper, it makes more sense for businesses to make the switch.


We hope that we were able to break down this complicated and technical topic into digestible bites!

When it comes to SIP calling, one simply cannot deny the many benefits and improvements it brings about to a business.

Software such as Novocall even incorporates it into its system to make the communication experience even smoother.

To find out more about how such technology can help your business, get started with us today!

Picture of Nigel Seah
Nigel Seah

Nigel is a Digital Marketing Executive at Novocall where he specializes in SEO. Prior to this, he had written for several SaaS companies including Workstream and the now defunct Hatchme. In his free time, he engages in strength training and is a lover of languages.

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