UPDATE: ISDN days are numbered
what happens next?

With the progressive rollout of the NBN network, over the next 2 years or so ISDN will eventually be phased out. This means that all businesses using ISDN will need to transition to a VoIP based phone service.

Arrow offers a complete migration service so you can transition to the cloud seamlessly. Call us today to discuss the best option for your business or read more here »

The telecommunications industry loves its acronyms, but it can all be a bit confusing with so many different products and standards available to provide voice and data services. Luckily for you, Arrow is here to make things a bit clearer!

Two of the oldest methods for voice service delivery are public switched telephone network (PSTN) and integrated services for digital network (ISDN).


Also jokingly known as plain old telephone service (POTS), PSTN is the phone line most of us grew up with at home.

PSTN is delivered to the premises via twisted copper wire pairs (residential and business). Although the core network is almost entirely digital these days, the signal is still usually analogue from the exchange to the end user.


“Integrated services” refers to the ability to deliver a minimum of two simultaneous connections over a single line. ISDN has been very popular for businesses that want to transmit voice, data, video and fax and caters to large organisations so they don’t need to install and manage multiple PSTNs.

ISDN comes in two main forms:

  • ISDN2 (BRI – Basic Rate Interface) – ISDN2 is the entry level version made up of two 64 kbit/s voice channels and one 16 kbit/s signalling channel that add up to 144 kbit/s.
  • ISDN10/20/30 (PRI – Primary Rate Interface) – Delivered on an E1 (2048 kbit/s), this version can support up to 30 channels or simultaneous calls each at 64 kbit/s, a 64 kbit/s signalling channel and also a 64 kbit/s timing and alarm channel. The product is delivered as a block of 10, 20 or 30 channels depending on demand.

PSTN Features

  • Line hunt group – Set up a call flow between multiple PSTNs so that if a call isn’t answered it’s passed on to the next PSTN in that group.
  • MessageBank – A virtual answering machine to save messages from missed callers.
  • Fax duet – A dedicated line for incoming and outgoing faxes on the existing PSTN line.
  • MessageBank away – Leave customers a message when closed for holidays or moving office.
  • Calling number display – Help identify who is calling before you answer the phone.

ISDN Features

  • Additional number block – Assign up to a 100-number block of unique phone numbers to give to each staff member. Perfect for large organisations.
  • Extension level billing – Allows itemisation of call charges for each number in an additional number block (also enables calling number display).
  • Line hunt group – Same as PSTN you can group multiple ISDN2’s together to form a hunt group.

OK, so what can you do with them?

Both PSTN and ISDN can have their network access restricted to stop particular calls, such as mobile, international, 1300 etc., from being made. Each system has a range of added value products. Hopefully this gives you a better understanding of what each of these systems can do.