Guide to Multi-line business telephone systems

Multi-line business telephone systems are small scale analogue or digital telephone systems. They provide a basic range of features & functionality, and are cost effective, entry level, business phone systems for start ups and small businesses. This guide will help you understand how Multi-line business telephone systems work and whether they are the right telephone system for your business.

This is the main photograph for the article Guide to Multi-line business telephone systems. It shows a close up of a key system unit telephone. handset

Multi-line business telephone systems were developed to allow businesses to make and accept multiple telephone calls at the same time without the need for a telephone exchange operator.

These systems are based on analogue or digital technology and provide a range of basic business telephone features, such as auto-attendant, intercom and voicemail, and could be a cost-effective solution for start ups and very small businesses looking to invest in their first business phone system.

However, a significant issue that business managers should be aware of as they select a business phone system is BT’s switch off of all analogue and Integrated Service Digital Network (ISDN) telephone lines by 2025 meaning that most business (and residential) telephony will need to be carried out over the internet using Voice over Internet Protocol (VoIP) technology by that date.

Those business’s looking to future proof their business telephone systems will need to consider this major technology change in their decision making. We cover this issue in more detail in our article What is a business phone system?

If you believe that a more sophisticated business telephone system is right for your business then please read our Guide to PBX business phone systems and Guide to VoIP phone systems articles.

The business telephone systems industry uses a wide range of acronyms and technical jargon. To help you get a clear understanding of the technicalities we have provided a glossary of business telephone system terminology which you may wish to refer to alongside our articles.

In this article

What is a Multi-line business telephone system?

Multi-line (or Key System Unit) business telephone systems are basic analogue or digital business phone systems that provide multiple telephone lines to a business’s premises. Multi-line systems come in wired (KSU system) and wireless (KSU-less system) variants.

The core of the KSU system is the Key System Unit itself, a separate hardware unit or switching equipment incorporated into a master phone handset, that controls the switching between external and internal telephone lines (usually up to 40 internal extensions).

Multiple external telephone lines can be connected to the KSU and the business can have separate telephone numbers for each linked external line and internal extension, or a single telephone number with the KSU making incoming calls available to all internal lines, or to internal lines in descending order. Alternatively, all calls can be directed to a single handset and each call then needs to be forwarded by the operator/receptionist to the correct handset extension number.

Each handset will have a button for each line with an associated indicator light. A telephone line must be manually selected by the handset user to accept or make a call (calls are not automatically routed to the recipient as is the case with a PBX business phone system).

Employees can also select in-house extensions to speak to their colleagues.

Multi-line business telephone systems

This infographic shows how Multi-line business telephone systems operate.

Telephone created by Georgiana Ionescu, Server created by Chunk Icons and Antenna created by AS Design, from Noun Project.

The KSU-less system does not need a central KSU nor any additional infrastructure to be installed as they have all of the required technical parts included in the wireless handsets. However, as they are wireless, the handsets need an independent power source to charge up and operate.

Comparison of KSU and KSU-less Multi-line business telephone systems

We have compared the KSU and KSU-less business telephone systems against the following criteria:

Comparison of KSU-less and KSU business telephone systems

This table compares the KSU and KSU-less business telephone systems against a number of criteria.
  • Business size: KSU business telephone systems allow a maximum of c.40 handsets to be operational on the system. KSU-less systems are not scalable above c.10 handsets. Both of these systems are, therefore, only suitable for start ups and small businesses that are not looking to expand rapidly.
  • Costs:
    • Upfront and maintenance costs will depend on the number of external lines required, the number of extensions, the phone handset features and the complexity of installation (in the case of a KSU system).
    • Monthly running costs will depend on the business’s call plan with its telephone service provider, its call volumes and the nature of those calls (e.g. local v. international).
    • KSU phone systems are the cheapest business telephone systems on the market. KSU-less systems can start from less than £100 with KSU systems starting at c.£200, plus line rental.
  • Range of features: The KSU and KSU-less systems offer a basic range of business telephone features (such as auto-attendant, intercom and voicemail) but these are very limited when compared to other business phone systems.
  • Remote working: Not available with KSU business telephone systems.
  • Customisation potential: Both systems have limited customisation potential.
  • Scalability: Both systems have significant limitations as to how far they can be expanded within business with the KSU-less system being the most restricted at only c.10 handsets. The KSU-less system is easy to move to another location whereas the KSU system would need to be wired in at the new premises.
  • Portability: As the KSU-less system is based on wireless technology it is extremely portable within an office. As the KSU system is wired into the premise’s infrastructure it is very restricted.
  • Integration with business applications: KSU business telephone systems have limited integration potential.
  • System upgrades: As KSU systems have very basic features there is little opportunity for system upgrades.
  • Reliability, security & control: KSU systems are based on conventional telephone lines making them extremely reliable and secure methods of communication although the KSU-less system is prone to issues with interference. As these systems are located within a business’s premises, they do not rely any third-party provider to operate the system and are totally controllable by the business.
  • Disaster resilience: Any incident resulting in serious damage to the premise and/or restricting access by employees to a business’s premises will make the telephone system unusable.
  • Installation, configuration & maintenance: As the KSU-less phone system has its technology built into its handsets it is straightforward to install. A KSU business phone system will need to be installed and maintained by a business’s IT/Telecoms team or an external contractor.


Multi-line business telephone systems are a short term, cheap and reliable solution for start ups and small businesses, with limited communication needs and very tight budgets.

Features of Multi-line business telephone systems

Multi-line business telephone systems come with more features compared to residential telephones but are very limited when compared to other business phone systems. Multi-line business phone system features include the following:

This table shows the features of Multi-line business telephone systems.

Read our article Business telephone system features for descriptions of each of these phone system features.

Components of Multi-line business telephone systems

KSU and KSU-less systems will normally include the following components:

This table compares the components of Multi-line business telephone systems.
  • New installed hardware: A KSU system will come with a small on-premise unit or the swtiching equipment built into the master handset. A KSU-less system, by its nature, will not require any hardware other than its phones. The complexity and cost of a KSU system will depend on the number of external telephone lines, the number of internal handsets and the range of features required.
  • New wiring: A KSU system will need to be wired into the business’s premise. The complexity and cost of new wiring will depend on the number of new internal extensions required and the size and infrastructure of the business’s premises.
  • Phone handsets: KSU and KSU-less systems will come with the appropriate analogue or digital handsets. Handset costs will vary according to the features provided.
  • Service provider: A business will need to sign up with a business telephone line provider to provide the required number of external telephone lines (analogue or ISDN).
  • Installation & configuration: KSU systems will require installation, configuration and maintenance by a business’s internal IT/Telecoms team or external contractors. KSU-less systems should be able to be installed by employees.

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