3. Types of Telephone Line - Guide to Telecoms | Midshire

Types of Telephone Line

There are a number of different telephone lines available for organisations. Please see below a breakdown on each line type, what it is for and what it does.

PSTN Line (Analogue)

The Public Switched Telephone Network is the analogue network for all residential and business properties around the UK. Analogue lines, made from copper, are primarily used for voice traffic, franking machines as well as fax and PDQ machines. An analogue line can also be connected to a modem for a dial up internet connection, although, this technology is becoming obsolete.

Common Issues with PSTN (Analogue) Telephone Lines

In the early days, phone calls traveled as analogue signals across copper wire. Every phone call needed its own dedicated copper wire connecting the two phones. However, this technology proved to be increasingly unreliable.

There are many reasons why a phone line can go faulty, especially with the degrading quality of the outdated copper network infrastructure. The older the property is, the worse the situation is likely to be, especially if a high standard of workmanship was not performed when it was first installed.

Copper lines can be affected by the simplest of things such as the weather and the lines being caught in trees outside the premises on overhead cable infrastructure.

One surprising issue with Copper lines is theft. They are commonly stolen from telephone boxes and units. Copper thieves cause despair amongst the wider community every year by stealing copper telecoms and Internet cable out of local cabinets, leaving users disconnected from family and friends.


A Multiline is a group of analogue telephone lines with one telephone number.

The first line is known as the main exchange line, whilst the extra lines are known as secondary lines.

The additional lines are added to the main exchange telephone line so there’s no need for a new number. This can prove useful for a business with one telephone number, where the user requires more than one line to take calls on. Having a secondary line added to an existing PSTN (analogue) line means that if a call comes through to an engaged line, the call will simply be passed to the next available line and it’s the same on outgoing calls.

Theoretically, there is no limit to how many auxiliary phone lines can be attached to one telephone number. However, the size available is limited by your local exchange’s capacity.

This service is suitable for small office organisations.

Digital Lines

There is a growing choice of technologies for business telecoms and Internet provision, ranging from using a standard telephone line and dial-up Internet access to installing a digital ISDN (Integrated Services Digital Network) line. There are two types of digital telephony solutions: ISDN2e and ISDN30e.

The choice between using the standard PSTN (Analogue) lines and ISDN does not just come down to price. By using ISDN, businesses will be able to introduce new features in there company’s telephone system and make use of additional applications, which will enable the business to grow.


ISDN2 is a high-performance voice and data service; suitable for small businesses. An ISDN2 solution allows the user to make two calls at once.

ISDN2 is the equivalent of two digital telephone lines, otherwise known as channels. Two channels can work independently allowing you to make a call, use the internet or receive a fax at the same time.

As the connection is digital, there is the added benefit of increased clarity for voice calls at 64 KB per second in comparison to 56 KB with traditional analogue lines. As a result, larger data files and emails can be sent and received more quickly and securely.

If your business requires further connections further down the line you can add further ISDN2 circuits to the existing setup in pairs.

ISDN30 is an advanced digital solution for larger businesses. It is the equivalent of up to 30 analogue lines each of which work at 64kbps.

ISDN30e is a high-performance voice and data service for modern businesses. Their digital technology gives higher line quality; which allow for:

  • Rapid connection
  • Clear voice communications with other ISDN users
  • Ideal for networks linking two or more offices
  • Provides staff with individual DDI numbers

An ISDN30 system provides the capacity for between 8 and 30 continuous voice calls. These calls can be both incoming as well as outgoing. Each call made will require 1 channel. If your organisation needs more than 30 channels, you would need to order an additional ISDN30 System, which can present the same number from channels 31-60.

With an ISDN30 System you can have an unlimited number of DDI numbers (Direct Dial), allowing your customers and business contacts to dial directly through to staff members, without passing through a switchboard.

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Advantages and Disadvantages of ISDN

Advantages of ISDN

  • The main advantage of ISDN is its 64kbps speed. Getting a connection is almost instantaneous whereas with a modem it can take time.
  • 64kbps transfer speed in comparison to a 56kbps modem
  • Higher quality calls.
  • Cost efficient in comparison to numerous analogue lines.

Disadvantages of ISDN

  • Requires specific digital equipment to interface with.

SIP - Session Initiation Protocol Trunking

Traditionally, legacy telephone systems were the standard for transmitting voice traffic. However, with the emergence of superfast broadband and the need to always be connected to family and friends, voice over the internet was created.

The protocol created to transmit voice traffic over a data line is called SIP Trunking (Sessions Initiation Protocol).

A SIP Trunk traditionally is connected to a business’s broadband connection in order to connect with the service providers’ network. Furthermore, a business can replace traditional telephone lines with a VOIP telephone system connected with SIP trunks and routed over the internet.

Although SIP Trunking works best with a VOIP telephone system, it can however operate with a legacy telephone system through a SIP Gateway.

The SIP gateway can act as a bridge to convert a standard telephone connection into VOIP and vice versa. This can be more cost effective for a business instead of investing in a whole new telecoms infrastructure.

Carrier Pre-Selection (CPS)

Carrier pre-selection (CPS) is a tool which allows businesses and organisations to select an alternative service provider to route their voice calls, other than BT, in advance without dialling additional codes on the telephone or programming your on-site equipment to do this. Many businesses and organisations are unaware that by using a secondary supplier, substantial savings can be made on phone calls – whether local, national, and international or calls to mobiles.

Least Cost Routing, or LCR for short, is a procedure used to always select the least expensive telephone service provider when connecting a call. Least Cost Routing is typically based on the Call-by-Call function, allowing users to select the desired telephone service provider by dialing a specific access code. Automated Least Cost Routing is intended to eliminate the cumbersome manual selection of the least expensive.

Wholesale Line Rental (WLR)

WLR stands for Wholesale Line Rental. It is a method which allows alternative telephone service providers to rent access lines on wholesale terms from BT, and resell the lines to customers, providing a single bill that covers both your line rental and telephone calls.

Local Loop Unbundling (LLU)

Local Loop Unbundling (LLU) is a facility which allows businesses and organisations other than BT to install their own telecoms equipment in the local BT exchange.

These telephone service providers can then offer their own direct phone or broadband services to consumers.

There are now over 30 different companies providing unbundled services to homes and small businesses.

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