What is SIP Trunking?

Session Initiation Protocol (SIP) trunking is the digital method of making and receiving phone calls and other digital communication over an internet connection. The term trunk in SIP trunking refers to virtual phone lines that you can use to make phone calls over the internet to anyone with a phone number. SIP trunks not only carry voice calls, they can also be used for multimedia communication that combines voice, video and messaging services. SIP trunks are replacing analogue and ISDN phone lines, with many countries now scheduling in the termination of ISDN services.

What are the benefits of SIP trunking?

SIP trunking suits businesses requiring multiple phone lines, from sole traders and small businesses to multinationals with numerous global sites. If you make multiple calls or think your call volume will increase in the future, SIP trunking could offer a number of benefits for your business.

Cost savings

Traditional phone lines can be costly, especially if your company has multiple unused lines or if you are regularly making international calls. Businesses also have to cover hardware and maintenance costs. SIP trunking is generally charged on a per-channel basis, e.g. based on the maximum number of concurrent calls you expect to make. Some services provide flexible pricing, so you pay only for what you use (unmetered vs. metered). SIP trunk pricing generally consists of

  1. A setup fee
  2. A monthly rental charge for the broadband line
  3. A price per channel that may include a call allowance
  4. Actual call charges.

Whether you chose metered or unmetered will depend on the type of calls you make, e.g. domestic vs. international. 


If you have multiple locations and use the same carrier, calls between sites are usually free (you only pay monthly rates for the data circuits like broadband).You can scale your internet connection to suit your intended maximum call volume. The amount of bandwidth required is determined by the media type (voice, video, messaging) and the codec used. Fibre broadband provides very high bandwidth rates, meaning a high number of concurrent calls can be supported.

SIP trunks are sometimes run over dedicated data circuits, particularly for larger business. In this case the circuit supplier offers either a service level agreement that provides a minimum expected service availability target, or a quality of service guarantee that ensures voice traffic has a minimum guaranteed amount of bandwidth and priority routing, or both of these.

This means you'll remain in control of your business phone system even as you add new domestic or international locations. You can manage and control your own resources rather than rely on a service provider to add new infrastructure or connections.


SIP trunks can be more reliable than traditional analogue circuits. Firstly, it is possible to support number failover, so if the primary number cannot be reached it can failover to a secondary number, potentially at a different location. Secondly, SIP trunks, when routing calls, do not have a fixed route end-to-end like traditional phone lines. Voice over IP (VoIP) calls can take diverse routes to reach their destination, thereby overcoming network issues or outages that would have prevented analogue calls from connecting. It is also possible to install diversely routed broadband lines to provide resilience right up to the office. 

Real-time analytics and insight

Because SIP trunking systems are cloud-based, you get real-time access to analytics and data insights. You are able to instantly access call logs to measure usage and availability accurately. You can input the data to machine learning services to derive business intelligence. Statistics can be broken down to individual users, so you’re able to get a snapshot of each employee, showing their communications with customers or prospects. This access to real-time information helps your business improve employee performance and drive improvements in marketing, customer service, and sales. 

Centrally managed communication

SIP trunking enables teams to make and receive phone calls, video conferencing, and messaging from one central location. Previously, these services would have been spread across multiple places, making communications inefficient and expensive. A SIP trunking communications system is straightforward to manage once it has been set up. By unifying your communications system, all of your company's digital communications will be integrated into one platform. 

Call quality

SIP trunks are designed to ensure that your digital phone connections have ample bandwidth to make high-quality calls. As long as your private branch exchange (PBX) isn't overloaded, you can expect clear calls even during busy periods. With advances in artificial intelligence, we now have technology that automatically improves call quality and fixes errors like echoes and static. You shouldn't have to worry about call drop-outs or echoey connections.


Unlike older phone systems, your SIP trunks are dedicated connections for your calls. SIP providers monitor calls for unusual behavior as part of their service, meaning anything unexpected will be flagged immediately. It is worth speaking with your SIP provider about their specific security protocols.


SIP trunks are not tied to a local exchange, so it is now possible to have local numbers in any region regardless of your physical location. Also, if you move premises you can take the numbers with you wherever you move to.

How does SIP trunking work?

In the past, companies used ISDN circuits, or copper lines, that they physically installed on their premises. SIP trunking replaces these analog phone lines with a digital version. The process works by breaking up calls into 'digital packets' and sending them across a data network. We explain the various components of the SIP trunking system below.

SIP channels

SIP channels are the digital version of the traditional phone line. Each SIP channel allows two simultaneous calls—one outward and one inward. Unlike physical phone lines, you can add new SIP channels as needed without the need for wiring. 

SIP trunks

Multiple SIP lines, often more than 20, are grouped into SIP trunks. The SIP trunk acts as the go-between for a company's phone connections and its Internet Telephony Service Provider (ITSP). You can think of the SIP trunk as a virtual phone line that connects a home or business to a telephony service provider. It allows inward and outward communication between the corporate network and multiple other phone and data networks. For example, you can connect your PBX to the Public Switched Telephone Network (PSTN).

SIP protocol

Session Initiation Protocol are a set of communication rules for the making and ending of data connections over the internet. It enables a voice communication session online so that users can speak with each other over a phone line, video call, or messaging service.

SIP provider

SIP providers are businesses that provide SIP services such as voice calls, video conferencing, and instant messaging. A SIP provider groups one or several SIP connections and runs them through a company's on-premise private branch exchange (PBX). Those SIP lines are then run through a SIP trunk, with calls and other digital communications enabled over an internet connection. The choice of SIP provider is dependent on the services your business needs. All SIP providers offer SIP trunking to businesses with a PBX, but only few provide an Internet Protocol PBX (IP PBX) for companies that do not have an existing PBX system. This means businesses that have nothing but phone connections can start using SIP trunking.

What is the difference between SIP and VoIP?

SIP and VoIP technology are related, but they are not the same. Voice over Internet Protocol (VoIP) is the recognized term for all voice calls made using an internet connection instead of analog phone lines. It is a broad industry term for the technology that supports online voice calls. While SIP uses VoIP technology for its voice calls, it can also support multimedia services (video, conference calls, and messaging) over its protocol. SIP is a broader technology than VoIP.

What is the difference between SIP trunking and PRI trunking?

A Primary Rate Interface (PRI) system, sometimes known as a PRI trunk, provides a physical connection to the PSTN instead of the virtual connection offered by SIP trunking. PRI uses copper wire to create up to 23 voice channels using a circuit-switched model.

What features should you consider when choosing a SIP trunking service?

Different SIP providers offer varying levels of SIP trunking service quality. Here are some features that you should consider before choosing your new SIP trunking provider. 

Real-time data

It is worth checking how much data your SIP provider offers and making sure you can view all of it in real-time. The more information you have at your disposal, the more informed your decision-making can be. Leading SIP providers like AWS offer a suite of transcription and machine-learning tools to turn recordings into actionable insights.


It is essential that your SIP trunking provider offers a high level of security and authentication. Leading SIP providers should be able to use encryption to secure your phone calls and other digital services.

Number management

If you are changing providers, you should be able to port across all your existing phone numbers while adding new connections and numbers from a central platform.

Call metrics

Before joining a new SIP provider, it is essential to find out what information is available about the calls you'll be making. Many SIP trunking providers will offer information about call completion percentages and voice quality.


Many providers offer services that can be customized to fit your existing business needs. Ensure you know how much you can customize your service before choosing a SIP provider. Your SIP provider should offer video conferencing, instant messaging and other communication features you require.

How can AWS help with your SIP trunking?

You can join thousands of other businesses using SIP trunking with Amazon Chime SDK. AWS customers use Amazon Chime SDK to route phone calls to multiple office and call center locations based on the caller's intent, number, and several other call contexts. 

Amazon Chime SDK offers a number of features, such as improving your understanding of communication sessions using AWS machine-learning capabilities. You'll also be able to innovate rapidly using AWS's communication building blocks, freeing up your teams to deliver improved customer service. 

Other features include:

  • Call recording and analytics
  • Proxy phone sessions
  • Phone number management and porting
  • Call-health statistics
  • Security and authentication
  • Quality and reliability
  • Cost-effective network access
  • Machine learning applied to call analytics
  • Natural language transcription services
  • Video capabilities

Begin using SIP trunking with Amazon Chime SDK by creating a free AWS account today!

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