SIP trunking and VoIP are two extremely popular choices of communication for businesses, however, it can be difficult to understand the differences between the two and which is the right choice for your company.

Our telecoms guide is a useful resource to start with, and below we go into detail about what exactly SIP and VoIP are, how they work and the main differences between them.  With over 40 years in telecoms, we’re in a position to provide you with the best solutions – just get in touch with your enquiry.

What is SIP?

SIP (Session Initiation Protocol) trunking is a method of communication that uses an Internet connection, to deliver voice rather than a traditional phone line. SIP allows users to communicate across the world using voice, data and video over a single connection.

It is a preferred method of communication by larger businesses as it is easily scalable and cost-effective. Delivering over SIP enables businesses to benefit from investment in large data connections as, using SIP, they also enable voice to be delivered over the internet (VoIP).

What is VoIP?

VoIP or Voice over Internet Protocol, is a term that refers to the transfer of voice communications over the Internet, rather than a traditional phone line. VoIP however, differently to SIP, is only able to initiate voice communications, not multimedia too.

VoIP tends to be used by small to medium-sized businesses, with a dedicated circuit for voice connectivity. VoIP is often used to support hosted telephony which is easy to budget for on a monthly basis at fixed costs over an agreed term.

Key differences

Whilst you may now understand what SIP and VoIP are, you may not know the exact differences between the two technologies. Below, we have put together some of the key differences, to help you understand which is the best option for your business.

How they work

Whilst they both use the internet to make phone calls, there is one main difference in the way that SIP trunks and VoIP work.

VoIP can only carry voice communications, so you can only make voice phone calls to other users, whereas SIP trunks can carry all forms of media, including video. This means you are able to have video calls with team members in other offices, or with clients.


There is one major difference in the costs associated with SIP trunks and VoIP that may impact your decision on which to choose.

VoIP tends to not have any upfront costs, whereas SIP trunking does have upfront costs associated with setting up the connection. For small businesses, these upfront costs may not be viable, so VoIP would be the best option.

Both options are cheap to upscale, adding extra bandwidth and SIP trunks being extremely cost-effective. VoIP is also great for budgeting, as each bill is the same each month, based on your initial connection choice with no surprise fees.


As stated above, upscaling is quick and easy on both options. Extra SIP trunks can be easily added or removed, depending on your staff levels. This means if a team member suddenly leaves or if you are experiencing rapid growth, you don’t need to worry about unused phone lines or waiting for new ones to be installed.

Who are they best for?

VoIP tends to be preferable to smaller and medium-sized businesses, this is because it can be supplied as part of a hosted telephony solution which offers predictable monthly billing that is easy to budget for with minimal upfront costs and easy scalibility.

SIP trunking is usually favoured by larger businesses, this is because it can easily deal with large communications, especially multimedia and can share the same connections as data traffic, with rules applied to ensure voice quality. Smaller companies tend not to prefer SIP trunks as they can be quite difficult to manage as they often do not have the IT departments and resources to manage the connections.

ISDN switch off

In 2025, the ISDN network is going to be switched off. This means that anyone on an existing ISDN network will need to find alternative communications, with SIP trunking and VoIP to support hosted solutions being the two main alternatives available to businesses.

In April 2017, BT Openreach announced that they will be switching off their existing ISDN network in 2025 (and will no longer have ISDN lines available for purchase from 2020), as they focus on a single IP network. ISDN stands for Integrated Services Digital Network and provides a high quality voice circuit with the ability to add features such as DDI services that are not available on analogue lines.

As a result, businesses will need to look to alternatives for their communications – keeping personalised numbers, managing a high number of concurrent calls, audio conferencing – and many see the switch as an opportunity to improve their telephone systems and even save money.

The two natural alternatives to ISDN are SIP Trunks and Hosted VoIP systems, both of which are advanced telephony options already used by small, medium and large businesses. Southern Communications offer an ISDN replacement service and can advise businesses on the best upgrade options, on a bespoke basis, based on your current infrastructure.

To help you know your options, what do SIP and VOIP mean and which is right for your business?

What is SIP?

SIP stands for Session Initiation Protocol and enables voice to be carried over the internet, rather than a traditional phone line. SIP is set-up and uses SIP trunks to enable voice traffic to be sent over a data connection without loss of quality .

To use SIP, a business is assigned a virtual SIP address and will access the system with handsets and other internet enabled devices. Once you’re using SIP, new channels can be created and removed at any time to meet demands and reflect your capacity or seasonal trends.

Main benefits include:

  • Affordable monthly rental costs
  • High definition voice quality
  • Number of lines based on users
  • Ideal for office moves/relocation

What is VoIP?

VoIP is Voice over Internet Protocol and can be accessed using SIP Trunks on an on premise system or via a cloud based hosted telephony system, where your telecommunication system is hosted off-site, rather than in your office. As it’s all in the cloud, the system is maintained by a telecoms provider, updates are carried out for your and the system can be accessed using multiple devices, such as smartphones and tablets.

It’s already a very popular alternative to ISDN and once on VoIP, your communications are future proofed.

Main benefits include:

  • Cost savings over ISDN
  • Advanced telephony features
  • Scalability and flexibility


Both SIP and VoIP are ideal alternatives to ISDN and will upgrade your telephony system to be flexible, advanced and more cost-effective.

As both can be tailored to your usage, you don’t need to pay for more than you need or use – which is why it’s a good financial decision to switch before 2025 as you will benefit from cost savings for an additional seven years.

As both SIP and Hosted VoIP systems work for small, medium and large businesses, it will depend on your specific setup, budget, and needs. Here is an overview comparison of the two:

On premise?Requires an on premise systemCan be used as on premise or a hosted system
ExpenditureCapex expenditure (upfront)Opex expenditure (monthly subscription)
MaintenanceOngoing maintenance Maintenance included in monthly fee
UpgradesNot includedIncluded
TrainingMay be an extra charge depending on your provider May be an extra charge depending on your provider
ExpansionMay be costlyNo upfront cost

– Free internal calls

– Simple process to add or remove SIP trunks

– Secure connection

– Add further UK numbers at any time

– Great return on investment

– Advanced features

– Ideal for multi locations

– Does not require a dedicated staff member onsite

– Provider handles installation and upgrades

– Secure connection

– Add further UK numbers at any time

– Easy and fast enhanced features

– Simple relocation process

– Advanced features

– Ideal for multi locations


– Loss of Internet results in loss of phone service (can go to voicemail or routed to mobile)

– Will require a maintenance contract

– Cancellation fees may occur

– Voice quality relies on Internet connection

– Loss of Internet results in loss of phone service (can go to voicemail or routed to mobile)

– Cancellation fees may occur

SIP would be the best option if:

  • Your preference is upfront cost
  • You don’t expect your telecoms requirements to change over the years
  • You have a lot of users on just one site
  • You like to keep in house resource to maintain your telephone systems

VoIP would be the best option if:

  • You do not want to incur high upfront costs
  • You do not have additional IT resource to manage the system
  • You have remote offices and mobile workers
  • You’re a small or medium sized business

Get in touch

If you would like to find out more about the best option for your business, or would like to discuss your requirements, give our team a call on 01256 391 046 and we would be happy to help you.