In most business environments, telephone calls are typically handled by a receptionist or directed to specific extensions using an internal intercom system. The technology enabling this seamless communication, both within and outside the organisation, is called EPABX. This system allows users to answer, transfer or forward calls as needed. It facilitates internal communication between different departments or branches, streamlining workplace connectivity.
What is EPABX?
EPABX stands for ‘electronic private automatic branch exchange’.
When an employee, say Arun, wishes to call another colleague, Bharathi, within the same office, the EPABX system follows a structured procedure. Arun picks up the phone, completing an electrical circuit that sends an off-hook signal to the EPABX. The system responds with a dial tone, indicating that it is ready for input. Upon dialling the extension number, for example 104, the EPABX control unit identifies the corresponding line and connects Arun to Bharathi using its internal switching matrix.
For external calls, the procedure differs slightly. Arun begins by dialling an access code (commonly 0), followed by the external phone number. Instead of routing the call internally, the EPABX locates an available trunk line linked to the public switched telephone network (PSTN), which is the local telephone exchange. The dialled number is transmitted, and the call is connected, with voice signals passing through the EPABX to the external line.
If all trunk lines are occupied, the caller may hear a busy tone, indicating that no external lines are currently free.
How are incoming calls managed?
Incoming calls are managed differently based on the EPABX system configuration. The PSTN routes the call to an available trunk line on the EPABX. Older systems require a receptionist to transfer calls manually, but modern automatic EPABX systems enable callers to dial extensions directly.
Advanced digital EPABX setups can be programmed to play an interactive voice response menu, prompt callers to enter an extension or automatically route calls to predefined numbers, such as a reception desk. The switching unit then connects the caller to the desired extension without human intervention.
How does the switching mechanism work?
The switching mechanism is the most critical component of an EPABX system: it directs calls to the correct destination.
In early systems from the 1970s and 1980s, electromechanical switches such as crossbar relays were used. These worked similarly to adjusting a ceiling fan with a regulator, where turning a knob moved internal components to establish connections. When a call was initiated, an electromagnet would pull a copper strip, bridging two phone lines like a switch. This physical connection allowed voice signals to travel between extensions.
A basic electrical switch is like a tap. Turn it ON and current flows; turn it OFF and current stops. But telecom systems like EPABX use more complex switches.
The simplest variety is called ‘single pole, single throw’: it’s like a light switch. The ‘single pole, double throw’ switch is more complex: it has one input and two output choices, like a fan regulator that switches between speeds. An even more complex switch is ‘double pole, double throw’, which has two inputs and four outputs.
Call switching in an EPABX is like a railway yard. An incoming call is like a train arriving at a central station with numerous platforms. The EPABX is the control system, directing the call to the correct extension just like a railway point switches guide trains to the right tracks.
When a user dials a number, the EPABX activates the proper sequence of relays, connecting the caller’s line to the recipient’s line, much like aligning railway tracks to allow a train to proceed smoothly to its designated platform. When the call ends, the relays are reset, preparing the system for the next connection.
Another way to visualise this switching process is as a multi-level network. A main incoming line can split into four branch lines, like platforms 1 to 4. Each of these branch lines can further sub-branch into sub-extensions, for example, 11, 12, 13, 14 under line 1; 21, 22, 23, 24 under line 2; and so on. If a call is intended for extension 104, the switcher activates the corresponding combination of relay switches in that order, completing the circuit to the correct line.
What switching techs is used today?
With the growth of electronics in the 1980s, electromechanical switchers were replaced with electronic devices. A more advanced digital electronic system converted each voice signal into digital by using Pulse Code Modulation (PCM). It further utilised Time Division Multiplexing (TDM), wherein each voice channel is assigned a time slot, enabling multiple intercom users to use the system simultaneously.
With the advent of internet technologies, Voice over IP (VoIP) technology is utilised in modern digital PBX systems. Just as the IP address directs your email to the recipient, voice or multimedia communications are transmitted over the internet to the designated telephone instrument.
The evolution from electromechanical relays to digital switching has significantly enhanced EPABX systems. Today’s EPABX technology integrates seamlessly with modern communication tools, supporting features like voicemail, call recording, and automated attendants.
T.V. Venkateswaran is visiting professor, IISER Mohali.
Published – July 02, 2025 08:30 am IST
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