June 22, 2023
SIP Trunks vs Cloud PBX - Differences & Similarities

SIP Trunks vs Cloud PBX - What Are The Differences & Similarities?

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The telecoms industry is a nightmare for acronyms, shorthand and different technologies that can make life very confusing when choosing a new telephone system for your business.

SIP Trunks and Cloud PBX are just two of these terms, often confused with one another, and used (incorrectly) interchangeably. There are quite a few differences and similarities between the two, which we'll explore in this article. 

Differences Between SIP Trunks & Cloud PBX

While both Cloud PBX and SIP Trunks are modern technologies, they are very different. Here are some of those key differences…


The main difference between SIP Trunks & Cloud PBX is that they’re actually different technologies SIP trunks carry calls from one destination to another (like telephone lines), and cloud PBX telephone systems deal with call routing and call handing.

SIP Trunks - Are ‘virtual’ telephone lines that use the internet to make telephone calls, as opposed to traditional copper telephone lines.

Cloud PBX - Is an internet based telephone system—hosted in the cloud, your office telephones and home workers connect to your cloud PBX via your internet connection.

Cloud PBX Vs SIP Trunks Diagram
Cloud PBX Vs SIP Trunks Diagram

Almost every Cloud PBX uses SIP Trunks to connect the callers to one another. However, it is the Cloud PBX that controls where the call goes, plays voicemail, enables call transferring and other features that businesses take advantage of.


SIP Trunks and Cloud PBX’s are used different by the user. SIP Trunks sole job is to carry call traffic from your phone to the end users phone. SIP Trunks use the SIP (Session Initiation Protocol) to make this happen.

A Cloud PBX sits on the internet between your business and the caller, and all calls are routed through it. Cloud PBXs are used for a huge variety of reasons, as they are highly customisable and configurable.

Billing and Cost

SIP Trunks are (usually) billed on a per-trunk basis. Each ‘Trunk’ can carry one call at any one time, so businesses need to work out how many simultaneous calls they need, and then base the number of SIP Trunks from that.

Cloud PBXs vary in billing, but a good majority are charged per-user. If your business had 5 members of staff, then they would have 5 licences, one for each user to make and receive calls. There are some (3CX, for example, which are often allocated as an overall system, rather than per user).

Similarities Between SIP Trunks & Cloud PBX

Although SIP Trunks and Cloud PBXs are different technologies, they do have a fair few similarities. Here are a few of our top picks…

Replacement for ISDN

Both SIP Trunks and Cloud PBX telephone systems can serve as a replacement to traditional ISDN. ISDN is the legacy technology that the UK used to use to delivery voice calls to callers. Typically, a business would have multiple ISDN lines in the form of ISDN2 or ISDN30.

In 2025, Openreach are switching off the UK’s ISDN infrastructure—mainly because it is outdated and no longer fit for purpose. Even by the end of 2023, Openreach won’t accept any more traditional ISDN or analogue orders.

We have a complete guide on the ISDN switch off if you’d like to learn more.

This is why many businesses are choosing to switch to SIP Trunks or Cloud PBX telephone systems, as they don’t rely on ISDN, and therefore offer far more future proof telephony.

Great Way to Reduce Business Running Costs

Migrating from traditional telephone lines or ISDN to SIP Trunks or a Cloud PBX will likely decrease your businesses running costs. By moving to these new technologies, your can consolidate multiple costs (all of your lines) into one.

Our team estimate up to 90% savings against by moving away from a traditional telephone system to a Cloud PBX. Here are just a few reasons why SIP Trunks or Cloud PBXs will save your business money…

  • No maintenance contracts
  • 5000 Free UK minutes per month per user, eliminating call charges
  • No costly PBX hardware to lease or purchase
  • Remove multiple telephone lines and send all calls down your broadband connection

Benefits of SIP Trunks

Moving to SIP Trunks unlocks a world of benefits for your business. Here are a few of our top benefits of SIP Trunks:

1. Get ready for the ISDN switch off

Moving to SIP gets your business ready for the 2025 ISDN switch off—which is something that all businesses will need to do in the coming years.

2. Improve voice quality

SIP Trunks offer High-Definition voice, making calls clearer and better for you and callers.

3. Eliminate call charges

Our SIP Trunks come with 5000 free UK minutes per month per channel, which helps you virtually eliminate UK call charges.

Benefits of Cloud PBX

Just like SIP Trunks, Cloud PBX telephone systems offer a huge array of benefits to your business, including…

1. Reduced cost compared to an on-premise solution

Cloud PBX’s are significantly cheaper than their on-premise counterpart. With savings on equipment, maintenance, deployment, upgrades and more.


2. No on-site hardware to maintain

Cloud-based PBXs don’t have any on-site hardware, which means you don’t need to fork out to maintain or keep them updated.

3. Free upgrades and ongoing support

As with all cloud-based software/technologies, our Cloud PBXs are covered by free upgrades and ongoing technical support from our in-house team.

SIP Trunks or Cloud PBX - Which is Best for my Business?

If you’re looking to upgrade your business telephone system, deciding which option is best can be a minefield. Below are some tips to help you decide between moving to SIP Trunks, or moving to a Cloud PBX.

SIP Trunks are best if…

  • If you have an existing telephone system that is paid for outright (ie not on a lease)

Hooking SIP Trunks up to your existing telephone system can save you a huge amount of money. This is often the most common option for businesses that own their PBX (LG/Samsung/Mitel etc), as it means they can get ready for the ISDN switch off, without needing to change your whole phone system over.

Moving your existing PBX to SIP Trunks also saves on call charges and line costs.

  • You utilise a cloud PBX that doesn’t have built-in SIP trunks (like 3CX)

Bigger businesses and SMEs often shy away from Cloud PBXs as the ‘price per user’ model can make it expensive. If your business is looking to move to the cloud, but doesn’t want to pay per-user, then 3CX is usually the best option.

We usually recommend moving to 3CX for businesses over 20 users, as it is far more cost effective than Cloud PBXs.

A Cloud PBX is best if…

  • You require flexibility

By their very nature, Cloud PBXs are highly flexible. They enable home working for all of your users, with seamless transferring and call controls between everyone in the business (with. No need for complicated SD WAN or VPN setups!).

We usually recommend moving to a Cloud PBX if your business likes to have the flexibility to work anywhere, whether that’s in the office, at home, or on the road.

  • You have an existing on-premise telephone system

One of our most common deployments of our Cloud PBX is to businesses looking to do-away with their existing, on-premise PBX. Brands such as Mitel, Samsung and LG have been pulling out of the PBX market left right and centre as the industry moves to the cloud.

This puts strain on businesses who rely on their on-premise solutions for their telephony. We’re seeing lots of businesses upgrade from their on-premise PBX to a Cloud PBX.

With so much jargon in the telecoms industry, it can be difficult to cut through the noise. Our team are here to help, if you have any questions, or you’re just looking for some friendly advice, get in touch today!

If you have any questions, or would like some more information about how Bytes Digital can help your business, email hello@bytesdigital.co.uk give us a call on 03331 301 021.

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