Preparing for the PSTN Switch-Off: Integrating Digital Telephony with PA Systems

Even if you haven’t read about the PSTN switch off in the media, there’s a pretty good chance that you’ve had a call from a comms provider trying to sell you a new digital telephone system. PSTN is the Public Switched Telephone Network, and in 2025 it’s going to be switched off spelling the end of old analogue phone lines. 

Being just around the corner, you can appreciate these companies are trying to get you ahead of the game. However, they can overlook that within businesses, telephones are often linked to Public Address, or Tannoy Systems. 

Needless to say, in many respects the digital world offers superb flexibility and many benefits compared to the old analogue world. Unfortunately, newer digital telephony devices usually aren’t natively compatible with sound systems like the trusty analogue systems were.

What’s it all about? Why don’t they work? What can we do?

Links to public address (PA) systems are normally made via a dedicated connection on an exchange, which is a telephone control unit. Generally, the exchange is a control box that switches wires, routing calls from one handset to another within a business. Older signals were analogue, making it easy to feed into an amplifier via readily available and simple to fit interface boxes which sat in between the telephone and sound systems. 

With digital telephone systems, however, the exchange is usually a virtual environment running in the Cloud. Even when there is an “exchange” on site, it will be a computer that sits on the internal network of the business. Modern desk phones too are essentially small computers connected to the network, just like printers, scanners, and other computers. It is not possible to feed the 1s and 0s of digital audio straight into an analogue amplifier with the desired outcome of a Tannoy. 

However, within a digital telephone system there is a protocol, or set of rules which can fundamentally be considered a language, that the telephone handsets use to communicate with each other. This is normally a session-initiated protocol, or SIP, which can be utilised to provide a connection to a sound system. A digital interface unit which understands the SIP, when connected to the network can be programmed to become an extra phone number on the telephone system. When that number is called, the digital interface unit will answer the call, convert the incoming digital signal into an analogue audio feed and output it to a sound system. 

Thanks to the wonder of this technology, it’s possible to combine the old with the new and keep the analogue systems relevant in a digital world. Overall, as the audio world moves into the digital domain, you can have a completely flexible network-based sound system that would work seamlessly with digital telephony, but we’ll cover this in a future blog post. 

What next?

The uptake of these new digital systems is going to be massive in 2025 and there’s an expectation within the industry for a backlog of work. To make sure you’re not caught out give us a call for advice on how best to link it to your PA/Tannoy system. 

Discover The Benefits

With the PSTN Switch-Off fast-approaching, preparation is key. For further advice or guidance, feel free to get in touch!

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