Overview of business telephone lines

Every business needs a phone line to operate. Whether this just means having a line installed to provide data connectivity, or a full-fat ISDN30 line connecting your phone system, the need for connectivity is universal. There are many different types of phone line, though, so which does your business need?

The types of phone line PSTN

PSTN, or the Public Switched Telephone Network, refers to the network of old copper lines running across the
UK. Most PSTN connections are residential phone lines, where the user doesn’t require much functionality.

PSTN lines offer a few advantages for businesses:

  • Cost PSTN lines are relatively inexpensive to rent

Reliability Since it’s such a simple system that’s been in use for so long, PSTN lines tend to be very reliable.

Availability The entire UK is wired for PSTN connectivity, so you will be able to get a line for your premises no matter where you are.

Simplicity No equipment other than a phone is needed for you to get a PSTN.

line up and running. Simply plug in a phone (which takes power from the phone line itself) and start dialling.

However, PSTN probably isn’t a good idea for business use. Although there are a few benefits, PSTN’s disadvantages far outweigh these:

  • Capacity PSTN lines are only capable of carrying one call at a time.
  • Features PSTN doesn’t support many features that businesses use routinely, for example transferring calls to external numbers or auto-attendants.
  • Numbers PSTN is provided on a strict one number, one line policy. It is not possible to have multiple numbers ring to the same PSTN line.
  • Longevity with the expansion of fibre and specifically FTTP, no more PSTN lines are likely to be laid after the ISDN switch-off in 2025.

For these reasons, PSTN lines are best left to be dedicated to a fax or franking machine, or simply for an ADSL data connection.


ISDN stands for Integrated Services Digital Network. Launched in the 1980s, ISDN allows for simultaneous data and voice connectivity over the traditional PSTN. ISDN comes in a few flavours in the UK ISDN2 and ISDN30. BT announced recently that they will stop taking any new orders for ISDN connections in 2020, with a view to shutting the network down entirely in 2025 bear that in mind when choosing a phone line for your business.


ISDN2 is the lower capacity of the two ISDN line types in the UK. It has several advantages over PSTN services:

  • Capacity ISDN2 lines are capable of handling two calls simultaneously, making them more suitable for businesses than PSTN.
  • Expandability ISDN2 connections can be expanded up to 60 channels.
  • DDIs ISDN lines support the use of number ranges (DDIs) so you can have multiple phone numbers connected to the same physical line.

This is not to say it’s the best solution for businesses. ISDN2 has a lot of disadvantages that should be considered:

  • Cost ISDN is inherently more expensive than PSTN. Connections are charged per-channel. Each channel typically costs around the same as PSTN line
    rental, so the costs of ISDN rapidly add up.
  • Installation ISDN is complex and expensive to install. Installation usually costs several hundred pounds, and there’s a lead time of several weeks for an
    ISDN line to be connected.
  • Equipment ISDN lines require special equipment on the terminating end to work. You cannot simply plug a normal phone in to an ISDN socket and expect it to work“ in most cases you will need a PBX or router with a BRI (Basic Rate Interface) card installed.
  • Maintenance ISDN lines and equipment are expensive to maintain. Adding services, extensions or more capacity usually requires engineers to attend your site.

There are many cases in which ISDN2 or ISDN2e can still be useful to businesses“ for example ISDN is ideal for connecting cash registers or tills to a central location since it represents a constant data connection. This is especially important in areas where broadband connectivity is patchy.


ISDN2’s bigger brother is ISDN30. The main difference between ISDN2 and ISDN30 is the capacity“ where ISDN2 is limited to a maximum of 60 channels, ISDN30 comes with a minimum of 8 channels, and can be expanded as far as it needs to be. Each line can carry a maximum of 30 channels, but lines can be bonded together to give access to more channels if needed.

Since ISDN30 is essentially the same as ISDN2, the same advantages and disadvantages apply to it. However, it’s important to note that ISDN30 is cheaper per-channel than ISDN2, so larger businesses will probably want to install ISDN30 in the first instance to allow for later expansion.

It’s also worth considering the difference in protocol between ISDN2 and ISDN30“ where an ISDN2 system needs a BRI card to work, ISDN30 works in a slightly different way so needs a PRI (Primary Rate Interface) card installed in your PBX for it to function. Upgrading from ISDN2 to ISDN30 can therefore be very costly.


With ISDN’s days numbered, VoIP is almost certainly the right solution for your business. VoIP, or Voice over Internet Protocol, leverages your internet connection to deliver calls to your office. After the ISDN network is shut down, all new business phone connections in the UK will be provided through VoIP services. The advantages of VoIP over ISDN and PSTN are huge:

  • Cost VoIP is much cheaper for operators to run, so it’s a lot cheaper to install in your business. Per-channel costs are a fraction of those of ISDN, and call rates are usually much cheaper than the copper network.
  • Expandability VoIP systems, particularly those hosted in the cloud, expand with your business. New services and channels can be assigned to you by your provider very quickly.
  • Maintenance If you use a cloud-based PBX, you will never need an engineer visit from your provider. In most cases all you’ll need to do is just plug your phones into your existing network.
  • Features VoIP offers the most features of any of the main line types. Since VoIP systems are internet-native, they can be integrated easily and cheaply with a lot of the services your business already uses“ for example CRMs and email clients.
  • Lines Since VoIP isn’t geographically linked to your business, you can have numbers from any area in the UK and almost any other country ringing to
    your system. This can allow you to present an international presence, or appear local to your customers.
  • Connectivity Softphone apps let your remote and mobile workers log in to your PBX to make and receive calls. This increases their availability and reduces the cost of calls.
  • Survivability Hosted VoIP systems are not on your site, so if there’s ever a problem with your office you can simply plug your phones in at home or in
    another office to get back up and running.

Now, VoIP is brilliant, but it’s not quite perfect yet. Since it relies on the internet to deliver calls to your site, it is only as reliable as your connection. It’s also worth configuring your network to prioritise VoIP traffic if someone in your office uses all the bandwidth available on your line, you will find that calls either don’t get through to you, or they have bad quality.

This can be mitigated with router configurations but doing so is often quite complicated. For these reasons it may be prudent to install a separate ADSL or Fibre line dedicated to your VoIP system this would eliminate any risk of poor call quality or dropped calls but does present an extra expense.

A few years ago, we might have suggested that the best line type for your business would depend on what kind and size of business you run. However, VoIP has matured significantly recently, to the point where it is now hard to recommend ISDN. VoIP is now the default suggestion for almost all businesses especially with the ISDN switch off being announced.

Do you think you’re one of the few businesses who could still benefit from ISDN? Or perhaps reading about VoIP has convinced you to make the switch? Get in touch with us today, it’s not as hard as you think.

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