Whether you are looking in to your first phone system, or upgrading an old one, it can be difficult understanding the different types of lines. The number of connections required on a line can determine which line is better suited for your business, as well as the features you intend to use. So, let us look at the 4 main line types available.

PSTN Lines

PSTN stands for Public Switched Telephone Network, and is a traditional land line. This is the same sort of line you might have at home as well as in your office. It can provide basic features such as Caller ID and Call Diversion but is limited to a single telephone number no matter how many lines you actually have. Each line is provided on a separate pair of copper cables form the local BT exchange and the telephone number on the line is tied to that BT exchange. This type of line dates back over a 100 years and while the equipment in the exchanges has changed over the years the line is still pretty much the same analogue line as it was back in the late 1800s.


ISDN2 stands for Integrated Services Digital Network and is also known as Basic Rate Interface, or BRI for short and these are Digital Lines. As suggested in the name the line starts with 2 channels and further channels in pairs can be added to give as many channels as required. However, over about 10 or 12 channels most companies switch to ISDN30. Like PSTN, ISDN2 lines are provided on copper cables from the local BT exchange.

Both types of ISDN lines add features such as DDI (Direct Dial) numbers provided in blocks of 10 consecutive numbers. This is often used to give a dedicated number for each employee/department. ISDN also allows calls to be transferred to external numbers and includes Caller ID so users can see the number of the person calling them before they answer the phone.

ISDN2 lines have been around since the early 1990s; originally used for transmitting data and in the early days of the internet before broadband was available, but used extensively for the last 20 years or so for telephone calls for business phone systems.


ISDN30 is also known as the Primary Rate Interface, or PRI for short. While having the same features as ISDN2 the difference is the number of channels involved. ISDN30 has a minimum of 8 channels to be installed and can be increased 1 channel at a time up to 30 channels on a single connection but multiple connections can be added to give 60, 90 or even 120 channels. It is a perfect solution to a large business with multiple line requirements.

A site survey is often required before installing any lines and switching to a ISDN system, especially for an ISDN30 line. Although you can add additional lines at any time, if you know you will need them installed in the near future, it is easier – and cheaper – to install them all at once.

ISDN30 lines have been around since the mid 1980s and been very popular until the last few years when SIP has taken over.


SIP stands for Session Initiation Protocol and is the line type for connecting VoIP/Internet Telephony to a phone system. SIP uses an internet connection for the phone system to connect to the telephony network provider.

It provides all the features of ISDN including Caller ID and DDI numbers but importantly it’s not tied to any BT exchange, so you can have telephone numbers with any Local STD code and you can present different telephone numbers when you call out.

SIP is also considerably cheaper than any landline type and the quality of the calls is related to the quality of the internet connection. It also has greater flexibly and is better for Disaster Recovery solutions.

An understanding of your current phone system, and future requirements will be necessary to help determine which of the different line types and systems will suit the needs of the business.

At LTS we have over 30 years of experience so we can help you determine which system is best for your business, just give us a call today.