SIP trunking explained: Moving from PSTN to SIP trunk

SIP trunking is the modern way to manage business calls over the internet, replacing the old Public Switched Telephone Network (PSTN). With PSTN lines like ISDN BRI, E1, or T1 being phased out, businesses are moving to SIP trunking to reduce costs, gain flexibility, and improve call quality.

More and more businesses are making the switch to SIP trunks for their communication needs. This article explains what SIP Trunks are, the difference from PSTN, their advantages, and how you can make the move.

sip trunking diagram

What is a SIP trunk?

A SIP trunk is a set of virtual phone lines that allow businesses to make and receive calls over the internet. The term "trunk" refers to a group of phone lines, and “SIP” is the protocol used for making calls over the internet, also known as Session Initiation Protocol. Your provider supplies phone numbers and virtual lines, usually at better rates than the traditional carriers and with more flexibility and shorter contract durations.

SIP trunks are the modern version of the old T1 trunks. In the past, businesses would buy a T1 trunk from a telecom provider to connect to their phone system (PBX). Now, with SIP trunks, you buy from an Internet Telephony Service Provider (ITSP) and connect to a VoIP or IP PBX system.

How does SIP trunking work?

With SIP trunking, your business phone system is connected to a provider through your internet connection, allowing voice calls to travel over your data network and internet connection rather than physical copper lines. Each SIP channel acts like a virtual phone line, meaning that for every call you make or receive, one channel is used. If your business needs to handle more calls at the same time, you can easily add more SIP channels without installing new hardware. You can even keep your current phone numbers by porting them to new SIP services.

When someone calls your business, the SIP trunk directs the call through the internet to your phone system. The same applies for outbound calls, which are routed through the SIP trunk to connect with traditional phone networks.

With no hardware limitations and better scalability, SIP trunking makes it easier to manage your phone system entirely through your IP PBX or hosted service

Benefits of SIP Trunking for Business Phone Systems

Pros

  • Lower monthly Line & DID Rental – The monthly fee to have a number of lines installed at your office will drop significantly.

  • Lower call charges – The vast competition has driven down call charges significantly. Some of the offers even come with unlimited calling and you can also save on international call charges.

  • Better customer service – Provide better customer assistance by adding more geographical and international numbers. Quickly and easily add numbers to your SIP Trunk and terminate them on your PBX server – you can give customers more options to dial in at a significantly lower cost, even long distance. Customers can contact you easier and sales will increase.

  • Move offices and keep the same number – SIP Trunks are not bound to a location, so it’s easy to move offices without having to change your stationary or inform your customers. There is no longer any need to pay to forward phone calls to the new offices.

  • Eliminate VoIP Gateways – You can eliminate the need to buy and manage VoIP Gateways. All incoming and outgoing calls travel via IP. No extra conversion often means better quality too.

  • Leverage a modern IP PBX – Modern Unified Communications solutions will give customers increased productivity, mobility and boost sales. Connecting a PBX to SIP Trunk is much easier than via the PSTN. You can go on-premise or hosted, the choice is yours.

  • Flexibility – It is easy to add channels and cope with increased calls. A simple phone call will allow you to add channels, and often this can be done immediately. Compare that to the delay in having additional lines installed and then having to upgrade your old PBX to handle more lines!

  • Correct number of channels – You can easily choose the correct number of channels that you need.

Cons

  • Businesses in very rural areas may not get sufficient connectivity or bandwidth to support VoIP calls.

  • Quality of service, maintenance, and resilience varies between SIP trunk providers. So make sure you choose a Preferred or Supported one according to your PBX vendor.

  • Registration-based SIP trunks could be susceptible to hackers obtaining the credentials and registering the trunk to make fraudulent calls. However, there are ways to mitigate this using various security measures.

Choosing a SIP trunking service provider

When it comes to choosing the right SIP provider, there are a few important things to keep in mind. It's not just about picking any provider—there are several factors that can really impact your business communications and overall experience. Here's what you should consider to make sure you're getting the best fit for your needs.

  • Security – As SIP Trunks are exposed to the Internet, it is very important to have a well secured network and an anti-fraud system in place. The anti-fraud system must monitor the system and provide protection against call fraud.

  • Own network – Does the SIP Trunk provider run its own network or is it a rebranded service? There are quite a few resellers of SIP Trunks from other providers. Select a provider who has control over their service and network.

  • Competitive Cost – Costs vary widely between available offerings and some vendors will purposely try to overcharge. Be cost-conscious, but expect to pay a bit more for business class service, and choose a provider with prices based on how many concurrent calls you make rather than number of users.

  • Number Porting – Do you have the option to port your phone numbers? Ensure that you choose a provider who can port all the existing numbers – not all providers are able to do this for all regions.

  • Support – Does the provider offer support? And what is their availability? 3CX has a wide range of SIP trunk providers that have been tested and approved for use with the PBX so that they are fully supported by us.

Upgrading internet connectivity

Once you have selected your provider, consider a dedicated internet line for the SIP Trunk. Most firewalls are able to handle multiple WAN connections, and, considering the low cost of an internet line in most places, a separate connection will be the most reliable way to ensure the quality of your VoIP calls.

Some providers bundle their offering with a dedicated internet line. This keeps your voice traffic separate from your data traffic. Much will depend on the cost and your network infrastructure - check that your firewall is up-to-date and will be capable of handling VoIP traffic correctly.

Using an IP-PBX for SIP Trunks

Your old PSTN lines are likely to be connected into another old device, the hardware-based PBX system. Technically it is possible to buy a VoIP gateway that enables the old PBX to talk to the SIP trunks. But why not upgrade to a modern IP PBX and leverage the flexibility and modern features IP telephony can bring to your business phone system?

This allows you to take advantage of the cost savings, easy management, and productivity increases with full-scale Unified Communications that an IP PBX offers. 3CX PBX can be easily deployed on-premise with Windows or Linux, virtualized with VMware, KVM or Hyper V. Alternatively you can opt for a hosted solution managed by 3CX or hosted in your private cloud account with Google, Amazon, Microsoft Azure, Digital Ocean and more.

Are PSTN lines being phased out?

The major telecom providers are fast phasing out the old PSTN functionality and are moving customers to IP. Many European countries have already completely phased out ISDN with countless others to follow. The majority of businesses these days are already using modern IP phone systems and SIP trunks.

Phone companies like Verizon have phased out ISDN in the U.S. and in the UK, ISDN lines have significantly decreased. In 2025, the UK continues its gradual shutdown of ISDN services, aiming for a full switch-off by 2027. Other regions in Europe and APAC are on a similar path.

As a result, Session Initiation Protocol (SIP) trunking has increased dramatically, with the market value expected to reach US$35.5 billion by 2030.

Your questions answered…

We’ve covered some of the most common questions on SIP trunking below.

All businesses will benefit from a SIP trunking solution, but if your company handles a high call volume, frequently makes international calls, or wants to upgrade from an existing phone system to a more scalable and flexible solution, SIP trunking is a must. It gives you greater control, better call quality, and the ability to future-proof your communication system, all while saving money.

VoIP (Voice over Internet Protocol) refers to the technology that allows voice calls to be made over the internet, bypassing traditional phone lines. It converts voice into digital signals, transmitting them over IP networks. VoIP is a broad term that encompasses any voice communication over the internet.

SIP trunking, on the other hand, is a specific method used to deliver voice, video, and other unified communications over the internet using the SIP protocol. SIP trunks replace traditional telephone lines, allowing businesses to connect their on-premises phone systems to the public switched telephone network (PSTN) via the internet. A SIP channel refers to each individual virtual phone line used to make or receive calls, which allow you to handle multiple outbound calls or inbound calls at once.

PRI trunking relies on a traditional phone line system, where businesses use physical lines for handling calls. Each line can support a limited number of simultaneous calls. In contrast, SIP trunking is entirely digital, using your internet connection to handle calls without the need for physical phone lines.

IP PBX (Internet Protocol Private Branch Exchange) is a phone system that manages internal and external calls within a business over the internet or a local network. It handles call routing, voicemail, and other features. SIP trunking connects an IP PBX system to the internet, allowing it to make and receive external calls through a VoIP provider. While the IP PBX manages the calls, SIP trunking provides the connection to the outside world, replacing traditional phone lines. In short, IP PBX is the phone system, and SIP trunking is the service that connects it to external networks.

Further reading

Last Updated
This document was last updated on 4 July 2025
https://www.3cx.com/pbx/sip-trunking/