SIP Trunks Explained by the Pros

8th April 2021

What is SIP Trunking?

SIP, otherwise known as Session Initiation Protocol, is a type of voice over IP (VoIP) technology. It creates and terminates audio and video sessions with one or multiple parties in a hosted VoIP phone system.

How does SIP Trunking Work?

The ‘trunking’ element refers to phone line bundles shared by users. It is a virtual link between your PBX phone system and the standard phone network. Your SIP trunk provider connects multiple channels to your PBX system to allow you to make local and international calls over the internet.

SIP trunking can help businesses save money, but it requires informed decision-making to ensure you get the right number of trunks.

woman on voip

Who Uses SIP Trunking?

Any business can use a SIP trunk. You don’t have to belong to a specific industry to utilise this innovative protocol. It provides a convenient solution to streamlining operations and future-proofing your business.

If you already have an IP PBX system, it’s only a matter of time before your provider suggests SIP trunks. It will help you accommodate increasing call volume, engage with a growing international audience, and get better control over your communications channels.

What does a SIP Trunk Do for Your Business?

Cost Savings

SIP trunking uses the internet, allowing you to access clients or business partners at more reasonable rates. In most cases, you will be billed on a per-user basis.

Global Mobility

SIP trunking consolidates your voice and data into a single network, allowing for flexible scalability as you grow your business in new locations.


SIP trunking helps you avoid the typical problems of traditional phone systems, which include service interruptions due to weather events and damaged infrastructure.

Simple Management

SIP trunking works similarly to HTTP in that it has several administrative portals that make call routing easy and quick.

How is VoIP Changing the Telecoms Game?

SIP Trunking Explained: A Step-by-Step Guide for Businesses

Before you apply for a SIP trunk, you need to know how to calculate calls to give you an accurate idea of your SIP trunking requirements.

Step 1: Determine How Many SIP Trunks You Need

Although all types of businesses can use SIP trunking, the number of trunks to get often depends on your industry. If you’re in the telecom sector, you have higher SIP trunking needs than a manufacturing business. A good rule of thumb is, the more calls you handle, the more trunks you’ll need.

Step 2: Determine How Many Calls You Have Now

If you’ve received client feedback about busy and hard to reach lines, you likely need more SIP trunks than you assumed. Although your industry is the basis of your trunking inquiries, you have to review your organisation’s unique needs, particularly its average call volume.

Step 3: Determine Your Company’s Size and Growth Plan

There is a misconception that the larger your business, the more SIP trunks you need. However, the opposite is true. One of the reasons to get multiple SIP trunks is to help multiple employees take or make calls simultaneously. However, the more employees you have, the less likely they’ll be on the phone at once.

The question you should be asking yourself is, “how many calls per SIP trunk is advisable?” If your provider gives you 20 channels per trunk, your employees can take or make 15 simultaneous calls. If you’re planning on growing your operations to the point where you expect 100 simultaneous calls, you will need at least five SIP trunks to accommodate the volume.

Step 4: Calculate How Many SIP Trunks You Need

The Erlang B formula, also known as the Erlang loss formula, is used to calculate the number of SIP trunks required for a business. It focuses on the profitability of call losses in a queuing system. The formula gives you a good estimate that you can use in your decision-making process. You can use an online Erlang calculator to compute with ease and efficiency.

Frequently Asked Questions

What is the Difference between SIP and VoIP?

A hosted VoIP phone system describes internet-based phone services. It uses the internet to transmit voice packets from one party to another. A SIP is a protocol that transmits data sessions in VoIP applications.

How many calls can a SIP trunk accommodate?

A SIP trunk uses SIP channels to accommodate calls. A SIP trunk has multiple channels, meaning a single trunk can accommodate at least 15 calls simultaneously.

How many channels are in a SIP trunk?

A SIP trunk doesn’t have a limit to the number of channels it can contain. Depending on your needs, your SIP trunk can have as many channels are you want.