What is SIP? Session Initiation Protocol meaning & definition
SIP (Session Initiation Protocol) is a set of rules that phones and phone systems use to start, manage and end calls over the internet. It's used for voice calls, video calls and messages. It's also the reason your business phone works wherever you have a good connection.
That's the short SIP meaning. But what does it look like when a customer calls your office? And why should you care which protocol sits behind your phones? Let's break it down.
What does SIP stand for?
SIP stands for Session Initiation Protocol. Each word tells you something:
- Session: a conversation between two or more people. A phone call, a video meeting or a chat.
- Initiation: starting that conversation.
- Protocol: an agreed set of rules, so different devices understand each other.
Put it together and the Session Initiation Protocol definition is simple: a shared language that lets devices set up, change and end a SIP session. That's also the most useful SIP definition to remember. A SIP phone from one brand can call a SIP phone from another brand, because both follow the same rules.
How SIP calling works
Think of SIP as the receptionist of a call. It doesn't carry your voice. It arranges the call.
Here's what happens when you make SIP calls:
- You dial a number. Your SIP phone sends an invite to the other side.
- The other phone rings and lets your phone know.
- The person answers. SIP confirms the call is connected.
- Your voice travels over the internet as data. Another protocol (RTP) handles that part.
- One of you hangs up. SIP ends the SIP session.
During the call, SIP also handles changes. If you put someone on hold, transfer the call to a colleague or add a third person, SIP sorts that out too.
SIP and VoIP: what is the difference?
People often mix up SIP and VoIP. They're related, but not the same.
VoIP (voice over IP) is the general idea: making phone calls over the internet instead of old copper phone lines. SIP is the most common protocol that makes VoIP work. So when people talk about SIP VoIP or VoIP SIP, they mean internet calls that use SIP to set up and control the call.
In short, VoIP is what you do and SIP voice over IP is how most systems do it. Nearly every modern SIP telephony setup relies on it.
What is a SIP phone?
A SIP phone is any phone that uses SIP to make and receive calls over the internet. It can be:
- a SIP handset on your desk (also called a SIP IP phone or SIP telephone)
- an app on your laptop
- an app on your smartphone
The handy part is that SIP phones aren't locked to one supplier. Most well known desk phone brands support SIP. So when you switch phone system, you can often keep the phones you already have.
What is Session Initiation Protocol trunking?
Session Initiation Protocol trunking (SIP trunking for short) connects your phone system to the public phone network over the internet. It replaces old ISDN and analogue lines. Your team can call any landline or mobile number, and customers can still call your normal business numbers.
A SIP trunk isn't a physical cable. It's a virtual connection, so adding extra lines is a settings change, not an engineer visit. That matters right now, because the UK is switching off its old copper phone network. Many businesses are moving their lines to a SIP connection for exactly this reason.
What does SIP mean for your business?
Picture a dental practice with three receptionists. It's Monday morning and the phones don't stop ringing. One receptionist is working from home today. Another is on her lunch break.
With a SIP phone system, all three are part of the same setup. A call comes in. The receptionist at home sees it on her laptop and picks it up, just like her colleague at the front desk. The caller has no idea where she is, and that's exactly the point.
That's what SIP makes possible:
- You call from your desk phone, laptop or mobile with the same business number.
- You add or remove lines and users without new cables.
- You mix and match devices from different brands.
- You keep working when you move office.
How Xelion uses SIP
Xelion is a cloud phone system built on open standards, including SIP. In practice, that means:
- Use the SIP phones you prefer. Xelion works with SIP desk phones from well known brands. You set them up centrally in Xelion Admin, so new phones are ready to go quickly.
- Call from anywhere. With the Xelion Windows App, Xelion iOS App and Xelion Android App, SIP calling works the same at the office, at home or on the road.
- Keep your numbers. Xelion connects to SIP trunks, so you keep your business numbers and say goodbye to the old copper lines.
- See what's going on. You see who's calling, who's available and who's already on a call. You set up queues, call groups and opening hours yourself.
- Call from Teams. With the Xelion Microsoft Teams App, you make calls from Teams with your Xelion number and address book.
All of this comes in one licence, with no separate modules for the features you use every day.
