SIP trunking explained: Moving from PSTN to SIP trunk

The trusted old Public Switched Telephone Network (PSTN), with its Analog lines, ISDN BRI, E1 or t1 lines, are gradually being replaced by IP telephony. An increasing number of businesses are using more modern and flexible SIP Trunks.

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these days are already using modern IP phone systems and SIP trunks, although some smaller businesses may still rely on outdated technology.

Phone companies like Verizon have phased out ISDN in the U.S. and in the UK, ISDN lines have significantly decreased. In 2017 major telcos such as KPN, France Télécom, Deutsche Telekom, Telecom Italia, and British Telecom began to retire ISDN lines, with the UK announcing that their entire ISDN infrastructure would be retired by 2025.

As a result, Session Initiation Protocol (SIP) trunking has increased dramatically, with the market value expected to reach US$35.5 billion by 2030. Unlike a PSTN provider, the lines provided are not physical, but a service provided over your internet connection. Your provider supplies phone numbers and virtual lines, usually at better rates than the traditional carriers and with more flexibility and shorter contract durations.

This guide 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 the modern-day equivalent of a T1 trunk. Where in the past you would buy a T1 trunk from a Telco provider and connect it to your legacy PBX, today you can make a purchase from an ITSP (Internet Telephony Service Provider) and connect it to your VoIP / IP PBX. SIP trunks use the SIP standard. The name ‘trunk’ comes from the telecom world and it means, more or less, a group of phone lines.

Using this standard protocol, telecom service (VoIP) providers connect one or more channels to the customer’s PBX. Phone numbers and DIDs are linked to the SIP trunk and in many cases, numbers can be ported to the new service.

What are the pros and cons of SIP trunking?

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 phone calls come in 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 providers.

  • 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.

Selecting the right provider

The next step is to choose your provider. A few factors come into play when making this decision:

  • 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. Look for competitive rates, but ensure that you are getting business quality service. Be cost-conscious, but expect to pay a bit more for business class service.

  • 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.

How to upgrade the PBX to an IP PBX

Chances are that the old PSTN lines are connected into another old device, the hardware-based PBX system. This device is inflexible, difficult to manage and often expensive to maintain. Technically it is possible to buy a gateway that allows 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 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 and more.

Further reading