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 today

With seven years until the switch-off now is the time to act. Before 2025 support and services for the old ISDN system will reduce and the sooner a business upgrades to VoIP or SIP, the sooner it will benefit from higher quality calls and cost savings.

To find out more and arrange a switch to SIP or VoIP speak to one of the Southern Communications team today.