A telephone system is the combination of hardware, software, and network connections that lets people make and receive voice calls. At its simplest, it connects two people speaking in real time. In a business context, it also routes calls between departments, puts callers on hold, forwards messages, and increasingly handles video and chat alongside voice. The technology behind these systems has shifted dramatically in recent years, moving from copper wires carrying analog signals to internet connections carrying digital data packets.
How a Telephone System Works
Every telephone system has three core jobs: establish a connection between two parties, maintain that connection while they talk, and terminate it when they hang up. A component called the call control subsystem handles all three tasks, along with features like call forwarding and call waiting.
In traditional systems, your voice travels as an electrical signal along copper wires to a central switching office, which physically connects your line to the person you’re calling. This network of switches and copper lines is known as the Public Switched Telephone Network, or PSTN. It has been the backbone of phone service for over a century.
Modern systems work differently. Your voice is captured by a microphone, converted from an analog wave into a digital format, then split into small data packets. Those packets travel over the internet to the recipient, where they’re reassembled and converted back into sound. This is the core principle behind Voice over Internet Protocol, or VoIP, and it’s how the majority of new phone systems operate today.
Types of Telephone Systems
Traditional Analog (PSTN)
The oldest type still in use relies on dedicated copper wires and analog signals. Each phone line physically connects to the telephone company’s network. These systems are reliable and familiar, but they’re expensive to maintain, limited in features, and nearing the end of their lifespan. In the UK, sales of new PSTN services stopped in September 2023, and the entire network is scheduled for shutdown by January 2027. Similar transitions are underway in other countries as carriers move everything to internet-based infrastructure.
VoIP Phone Systems
VoIP replaces copper lines with your existing internet connection. Calls travel as digital packets over a data network, whether that’s your office Wi-Fi, a wired connection, or even a mobile data plan. Because VoIP runs on standard internet infrastructure, it’s significantly cheaper to operate and far easier to scale. Adding a new user typically means plugging in a phone or installing an app rather than running new wiring.
VoIP systems can still connect to the traditional phone network. A technology called SIP trunking creates virtual connections between your internet-based phone system and the PSTN, so you can call any phone number in the world. These aren’t physical cables. They’re software-defined links delivered by a carrier over the internet, replacing the bundles of copper lines that older systems required.
Unified Communications (UCaaS)
UCaaS platforms take VoIP a step further by bundling voice calls with video conferencing, team messaging, and presence indicators (showing whether a colleague is available, busy, or away) into a single cloud-based service. Rather than running separate tools for phone calls, video meetings, and chat, everything lives on one platform. UCaaS costs more than standalone VoIP, but for businesses that rely on multiple communication channels, it consolidates tools that would otherwise require separate subscriptions.
The Role of the PBX
If you’ve ever called a business and pressed “1 for sales, 2 for support,” you’ve interacted with a PBX, or Private Branch Exchange. A PBX is the internal switching system that manages calls within an organization, routing incoming calls to the right person and allowing employees to call each other using short extension numbers instead of full phone numbers.
Traditional on-premises PBX systems required dedicated cabinets full of switching hardware, trunk lines connecting to the phone company, and extension lines running to every desk. The PBX cabinet acted as the brain, containing a switchboard for call handling and controlling features like call queues, voicemail, and automated menus.
Cloud PBX replaces all of that physical equipment with software hosted on the internet. A virtual switchboard handles call routing using customizable rules, sending incoming calls to different people or departments, running interactive voice menus, and managing voicemail. The only hardware on your end is the phones themselves (or computers running softphone apps). Hybrid systems also exist for businesses that want to keep parts of their existing phone hardware while adding cloud-based features gradually.
Common Features
Modern telephone systems offer a standard set of capabilities that go well beyond placing a call:
- Auto-attendant: A virtual receptionist that greets callers and directs them to the right department without a human operator.
- Call forwarding and ring groups: Calls can follow you to your mobile phone when you’re away from your desk, or ring multiple phones simultaneously so the first available person picks up.
- Voicemail-to-email: Voice messages arrive as audio files or text transcriptions in your email inbox.
- Call recording and monitoring: Calls can be automatically recorded for training or compliance, and managers can listen in for quality control.
- Call analytics: Dashboards that track call volume, duration, peak times, and other patterns to help manage staffing and response times.
- Presence indicators: Real-time status showing which team members are available, on a call, or away.
What You Need to Set One Up
For a VoIP-based system, the essential hardware is minimal: a modem and a router. The modem connects you to the internet, and the router directs data packets to the right devices on your network. If you’re transitioning from an older analog system, a VoIP gateway converts traditional phone signals into digital packets, letting you keep existing handsets during the switch.
Beyond that, the equipment is optional and depends on preference. VoIP desktop phones look and feel like traditional phones but connect to your network instead of a phone jack. They typically include a display for caller ID, buttons for hold and transfer, and built-in speakerphones. Headsets with a microphone let you handle calls hands-free, connecting directly to a computer. Many businesses skip dedicated phone hardware entirely and use software apps on computers or smartphones.
How Voice Calls Stay Secure
Because VoIP calls travel over the internet, encryption is essential to prevent eavesdropping. Two protocols work together to protect calls. The first secures the setup of the call itself, using a security certificate that acts like a shared secret code between the two devices, preventing anyone from intercepting or tampering with the connection. The second encrypts the actual voice data, generating a unique encryption key for every single packet of audio. Both need to be active simultaneously for a call to be fully protected. Most business-grade VoIP providers include these encryption options, though they sometimes need to be enabled in your account settings.
What It Costs
Cloud-based VoIP systems typically run $20 to $50 per user per month, with most providers offering tiered plans. Basic tiers start around $20 to $25 per user, mid-range plans with more features land at $30 to $40, and enterprise-level packages with advanced analytics and integrations cost $45 to $60. These subscriptions generally include the software, call routing, voicemail, and core features, with hardware purchased separately or rented.
Traditional on-premises PBX systems carry a very different cost profile: $800 to $1,500 per employee in upfront hardware and installation costs, plus $40 to $80 per line each month for ongoing service. Cloud systems eliminate that large capital expense, which is one of the main reasons businesses have been migrating away from traditional setups. For a small team of 10, the difference between a $15,000 upfront investment and a $250 monthly subscription is significant.