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Migrating from ISDN: What is SIP and Unified Communications?

An announcement in 2018 from major telecoms providers told us that ISDN and PTSN lines are to be killed off by 2025. This means that over the next few years, we will see this important legacy technology sink deeper and deeper into obsolescence, leaving a void that Voice over Internet Protocol (VoIP) solutions look set to fill. VoIP delivers feature-rich, efficient and customisable solutions that can be developed with the needs of specific businesses in mind.

As such, it is empowering for businesses to understand the options available to them, with two popular choices being SIP and an even bigger upgrade to a Unified Communications platform. Getting your head around this now will enable you to be best prepared for the inevitable shift from familiar analogue and ISDN connections and help you discern what will be most appropriate for your business when it comes time to join the migration.

Session Initiated Protocol (SIP)

SIP trunks transmit voice calls across an internet connection. It is possible to facilitate this solution with both digital and IP telephone handsets, meaning you won’t necessarily have to invest in a bunch of new hardware for the move to SIP to be a success. This is just one of the characteristics of SIP that makes it a low-cost, efficient solution for many growing businesses.

Advantages of SIP trunking

Moving from a PSTN to a SIP Trunk will bring about many benefits, both for your daily use and for your company. These include:

Low-cost calling

– in many cases, SIP calls are completely free of charge for local, national and mobile calls. This can save you as much as 60% in comparison to ISDN calling.

Cheap international calls

– even making calls to overseas contacts is far cheaper using SIP.


– as your business grows, the costs to add additional users are low, so you can scale up easily and cost-effectively.

Affordable rental

– the monthly cost of rental is usually low, and calls are often bundled in as well.

Greater telecoms opportunities

– extra features are available beyond traditional calls, including system integration and video calling.


– as ISDN networks are switched off, VoIP will likely become the most prominent telephony method to connect the UK and abroad.

You only pay for what you need

– with ISDN models and packages you frequently end up paying for various channels that never get used.

Disadvantages of SIP trunking

Reliant on the internet

– the quality of your calls will be determined by the quality of the internet connection the system runs on. You can combat issues related to this by having a convergent internet circuit. This is a system wherein a section of bandwidth is dedicated solely to voice traffic, ensuring the quality of the service is consistent. This approach also routes directly to the SIP provider, as opposed to routing over the open internet where it is more vulnerable and can be subject to congestion.

No internet means no phones

– if you lose your internet connection for any reason then unless there is a failover service in place businesses will find themselves without any working telephones.

A converged internet circuit is highly recommended when SIP trunking is in place. Quality of service for voice calls is extremely important, and by splitting the bandwidth between the phone network and general internet access, that quality is assured. Even when internet use hits particularly busy periods, the quality of phone calls will not be degraded.

Unified Communications (UC) platform

The first thing you need to know about a Unified Communications platform is that it doesn’t take the form of any one product or protocol. It is actually a marketing term that has been applied to the integration of several business communications services, including voice calls, video conferencing, speech recognition, presence information, instant messaging and more, with non-live communication services, including SMS, email and voicemail.

Rather than provide a set of separate products, one single interface is presented which provides a consistent experience for the user, no matter the device they are operating on. For instance, it is UC that enables voicemail or SMS to be accessed via a smartphone or email, and a response given in real-time or non-real-time format. Rather than being a rival product to SIP, UC is applied on top of SIP trunking like a building block to offer a more enhanced suite of products.

Advantages of a UC platform
Flexibility and functionality

– by making a range of communications tools available from any device, anywhere, UC is the choice for those who have a lot of communications channels and remote workers or stakeholders. Voice calls, video conferencing and emailing are made simple through one platform.

High compatibility

– using SIP trunking, UC platforms are highly compatible with a range of communication tools, cutting down on the need for various devices and software.

Cost effective

– SIP trunking enables VoIP, which caters for all your free communication needs. But UC platforms take that one step further by collating all communication tools into one place. Not only do you only have one service provider to pay, but workflows and queues are more easily managed, cutting down on lost productivity.

Improved support

– if you handle customer enquiries or provide support to a wide user base, then a UC platform enables you to collate all communication in one place, streamlining your responses more efficiently and improving the end user’s experience.

Disadvantages of a UC platform
Higher upfront costs

– migrating to a UC platform may be more expensive at the outset and incurs costs when training staff to use the new system. As with all business-wide changes, it may also see resistance from team members reluctant to get onboard.


– while UC platforms are designed to integrate a range of new software and technologies, there may be some issues with incompatibility when it comes to migrating from legacy software or integrating with project management or CRM software. There is also a lack of standardisation across UC platforms, which may cause issues between vendors.

More complex

– this isn’t a move to be taken lightly as a shift to UC can be time-consuming and complex.

A Unified Communications platform can considerably streamline your business communications, making it easier to connect with remote workers, stakeholders and customers, manage workflows, and it could even save you money. However, it is a complex process and requires deep consideration as to whether this is right for your business.

The important thing to remember is that if you choose to move on to SIP trunking, you will have the option to upgrade further to a UC platform as these are supported by SIP. By the same token, if you wish to go straight to a fully integrated UC, you’ll need to invest in SIP first. So whether you wish to migrate from ISDN to SIP now or are considering your options, please don’t hesitate to have a chat with one of our solutions experts. At Immervox, we are committed to helping optimise critical communications infrastructure at businesses of all sizes. With more than 20 years’ experience in the industry, we know what it takes to find the right solution suited for your businesses.

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