There are three main types of business telephone lines you can consider.  It is most likely that you will need to use at least two of the line types for your business requirements. We have summarised the key points for consideration below.

PSTN Business Telephone lines…

Analogue (PSTN) lines are the standard telephone line. They connect to a standard telephone handset or a phone system with analogue line ports. You will need a PSTN line to support any of the following devices or services:

  • Fax machine connection
  • Intruder or fire alarm systems
  • Emergency lift telephones
  • Analogue dial up modems
  • Remote access for maintenance support
  • Franking machines.

With  PSTN business telephone lines it is only possible to have a single telephone number per line. This could be restrictive if you were wanting multiple incoming call numbers for departme or want individual direct dial numbers to increase the efficiency of your incoming call handling.

ISDN Business Telephone lines…

ISDN lines offer the very highest level of resilience and quality available for connection to the telephone network.

Moving to ISDN lines provides a number of benefits for businesses. The two main ones that drive most businesses are:

  1. The ability to transfer incoming calls back out from your site to any external telephone number.
  2. The ability to have additional numbering options for incoming calls known as DDI (direct dialling inwards) ranges.

There are two types of ISDN Business Telephone lines available:


If your business needs between 2 and 8 voice lines, then ISDN2 is what you need.  ISDN2 is installed in pairs of channels so you can only operate with an even number of individual line rentals.They are powered from the local exchange with no power connection “on premises” required to operate them.


This option is available if you have a requirement for 8 telephone lines or more. ISDN30 business phone lines are presented on a single connection which can be upgraded to provide up to 30 telephone lines without the need for any additional cabling or engineering site visits. They can also be used with a digital switch or phone systems to manage your calls more effectively. ISDN30 requires power to be provided locally “on premises”.

VOIP… aka SIP Trunks….

A VOIP (Voice over Internet Protocol) service connection enables people to use the Internet as the transmission medium for telephone calls. It works by sending voice data in packets using IP as opposed to the telephone network.

One advantage of VoIP is that you can choose to have any UK STD code as your incoming number irrespective of geographic location. This can prove very useful for some businesses when relocating “out of area” as you can take your fixed line telephone with you anywhere.

Whilst it’s not ideal to use Broadband Internet access for SIP lines, if you do opt to use this type of connectivity then we would strongly recommend you have a separate broadband service for your phones to the one you use for your data.

The voice quality of speech via SIP trunks is very slightly lower than with ISDN lines. It is a similar quality to a UK mobile telephone call. The quality of service level is also slightly lower if you do choose to use a basic broadband connection for your Internet access method. This is because the response options to faults are traditionally slower with Internet providers than with telephone line providers.

We hope the above information is useful to you, if you would like help understanding your best options then please contact us on 0800 5053 350, alternatively you can visit our Business Lines & Calls page for more information or download our telephone systems Buyers Guide as useful reference material or even run a Quick Quote to obtain a budget cost for a new telephone system in less than 60 seconds.