The world of telecoms is full of confusing acronyms, especially when it comes to line rental. PTSN, ISDN, SIP, VoIP - what do all these acronyms mean? And which one is right for your business?

Here are the main differences between the big three: PSTN vs ISDN vs SIP – and what we think is the best for the future.


The Public Switched Telephone Network (PSTN) is the traditional copper wire telephone system, originally set up for analogue voice communication. It’s been in place since the late 1800s and although it’s been maintained and updated over the years, the technology underpinning it has essentially remained the same since its conception.

PSTN was once the most popular and reliable method of making and receiving voice calls. But now it's old-fashioned, and getting too expensive to maintain and operate.

Today, nearly everything is digital, so it’s hard to recommend a PSTN line, especially with more advanced and cheaper technology out there.


Back in the late 80s, Integrated Services Digital Network (ISDN) was the hot new successor to the analogue PSTN system and was born out the need for a more stable, secure connection for long-distance calling. It’s a digital version of a traditional telephone line, allowing for the transfer of voice and data simultaneously over PSTN copper wires.

It boasted clearer, higher quality calling than its predecessor, with less distortion and quicker sending and receiving of data. Although it still used the older lines, it was a welcome improvement from the often inconsistent and limited base PSTN.

Plus, with speeds of up to 128kbps, it was the ultra-fast internet the 1980s needed, setting the trend for more advanced (and exponentially faster) solutions later down the line.

But now even this technology is starting to show its age. New broadband technology has revolutionised voice communications and removed the need for costly, outdated analogue wiring, in favour of complete digital functionality. 


Lastly, we’ve got Session Initiation Protocol (SIP) and Voice over Internet Protocol (VoIP) which are quickly becoming the most popular modern replacements for PSTN and ISDN lines. VoIP is a broad term, and essentially means voice transmitted through the internet, rather than over traditional lines. SIP is the industry preferred protocol of achieving VoIP connections and extends your functionality by allowing you to send a range of multimedia data (e.g. messaging and video) over VoIP.

VoIP has set a new standard for voice communications. There aren’t any physical limitations, as calling is initiated over the internet, so your phone system moves with your business. It also simplifies features such as conference calls and call forwarding as everything is on one data network instead of dedicated phone lines. This makes it more efficient and scalable than any other system.

PSTN vs ISDN vs SIP: What's the Verdict?

With PSTN and ISDN lines only having a few years left in their lifecycle, it’s hard to recommend anything other than a VoIP/SIP telephone system. It offers high cost savings compared to traditional lines on calling and hardware, making it an attractive option for both small and large businesses.

Network flexibility is also a big benefit that a VoIP system provides over an ISDN or PSTN line. As its underlying network doesn’t need to be part of a specific technological layout, it's easily compatible with existing ethernet or Wifi configurations. It's also simpler to install, with SIP having the functionality to connect to most existing systems – removing the need for additional tech.

The flexibility of a SIP network makes it the best of the bunch and the most complete communications system that's currently available.

Are You Prepared for the ISDN Switch Off?

Soon, it won't even be a competition - because PSTN and ISDN are being switched off in 2025. That means if you're still using the old lines, you've got to start thinking about how you'll make the move to VoIP.

If you're looking to upgrade your business communications, Intercity Technology can help. We're experts in communication technology and can help you find, install, and manage the right solution for your organisation. 

For more information, don't hesitate to get in touch.