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ISDN Migration

As with all projects, the starting point is ‘why’. It is well publicised that Openreach will be ‘switching off’ ISDNs in 2025, with last orders in 2023.

So, the ‘why’ is quite easy. The switch off is to prevent resources, that could be used on modern infrastructure like Fibre, from being used to maintain an ageing network that is no longer fit for purpose; with parts in short supply and experienced engineers on the brink of retirement.

The next question is ‘when’. For this, there is not a one size fits all answer but your current contractual position should be the starting point, and if your circumstances are changing, it would be logical to move to new technologies.

The last question is ‘what to’ and there are three options: SIP Trunks to the telephone system, or, SaaS (Software as a Service) to the telephone system, or to the user’s handset, desktop and/or mobile device.

One element that should be considered is, as the world enters recession and with the move to cloud for telecom services, how much should be invested in telephone systems?

Things to be considered when planning the change

  • SIP Trunks are the logical replacement for ISDN, however SIP is not as resilient as ISDN, so, as always, business continuity planning is necessary.
  • SIP Trunks can be connected to your telephone system, either directly if compatible, or by the use of a gateway, so there is no change to the user experience, and you continue to maximise the investment made in your current hardware.
  • Delivering voice (via SIP Trunks) is not compatible with simultaneous data transmission, therefore separate connectivity is always recommended.
  • SaaS can be connected to a telephone system by using a gateway, as SIP Trunks.
  • SaaS deployment requires end users to have access to the internet.
  • Existing data networks can be tested for SaaS deployment.
  • About 90% of SaaS deployments have no physical handset and users either use a soft phone or an app on their mobile device, or a combination of both. This will probably alter the experience for the end user, but also provide them with a lot more features and benefits.
  • SaaS is generally feature rich, enabling productivity gains that are not available when using a traditional telephone system.
  • And lastly; between now and 2025, there is a huge number of telephone numbers to be ported from the current Public Telephone Network to the new providers. Currently, there are no approved industry processes that can speed this up, therefore the installation times will increase as we approach the ISDN switch-off.

Whichever solution is selected NSL will completely manage the process end to end.

Some of the Technical Detail

There many options when replacing an ISDN with SIP Trunks.

Initially it is good to understand that an ISDN30 is a 2Mb lease line (with 32 x 64Kb channels = 2048Kb) ‘nailed up’ connection from the network to the NTE at the user’s premises. The NTEs are a significant piece of equipment plugged firmly into the electrical outlet. There is a choice of SLAs and a well-practised issue resolution process. It is not unusual for an ISDN to operate for several years without an issue.

There are many choices of SIP trunks which breakdown into two categories: the choice of network that the SIP trunk connects to; and the data connectivity that is used to deliver the SIP trunk. The choices made in each of these categories will affect the resilience and cost of the service.

For the data connection lease lines are seldom used because they are cost prohibitive.

The ‘almost’ lease line is an EFM circuit which is a nailed-up circuit from the green box in the road to the user’s premises. An EFM generally has a substantial router with an external power supply and benefits from an SLA, synchronous speeds and uses the ethernet network core backbone. It is not unusual for an EFM to fail about once a year.

The most-used data connection is an FTTC which is a good quality broadband. The routers are broadband style with external power supplies, the speeds are not synchronous, and the service agreements are ‘Best Endeavours’. As a broadband the equipment will require rebooting regularly, and obviously uses the broadband core network which is subject to time of day spikes in usage. The connectivity is fibre to the green cabinet in the road and then a copper telephone line to the user’s premises.

The most cost-effective data connection is an ADSL circuit which is similar to an FTTC with the exception that the connectivity is to the local exchange and so speeds will vary depending on the location.

For all data connections the recommendation is to have an additional circuit for back up.

For SIP Trunks there are many suppliers with different options and commercials and contract lengths.

As with most things, the choice is resilience and cost.

The starting point for the selection is the connectivity of the network supplying the SIP to the PSTN (Public Service Telephone Network) and whether there are many points of contact or just one; the majority of cost effective SIP trunks connect via BT IPEX which, over the last few years has had a number of service issues. SIP services that are feature rich including back up usually have several interconnect points and attract a premium over a simple service.

As an alternative to SIP voice connectivity using SaaS is a very good solution as it does not require a nailed-up data connection so is inherently more resilient (please contact us for more details).

As at the beginning, there are many options and the decision is simply cost v resilience; NSL has had many years helping businesses with their comms and will make recommendations having understood the culture of your business.

Myth Buster

SIP calls are not free. They can be, if both parties are on the same network, but not always.

SIP is cheaper than ISDN. Yes and no; depending on whether you require a high level of resilience. For example, if you have 4 ISDN2 channels, SIP is likely to be more expensive, but also offers a host of new features.

SIP calls have better audio quality. Yes, if done correctly. When the PSTN was built by the GPO all those years ago, the engineers limited the transmittable frequency range. SIP calls are not usually limited, giving a greater range and depth of speech. Some SaaS services have HD voice for the best quality.

To find out more please contact us


  • Migrate before the switch off in 2025
  • We can advise which solution is best for your needs

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