What Is IP Telephony?
IP telephony is a way of providing telephone services over networks that use Internet Protocol, the same protocol that carries data across the internet and most office networks. It replaces the dedicated phone lines and switching equipment of traditional telephony with software and data connections.
The term is broader than it first sounds. IP telephony includes:
- Voice calls: Calls between IP phones, apps, and computers, and to ordinary landlines and mobiles.
- Video calls: Face-to-face calls over the same network and devices.
- Voicemail and messaging: Voicemail delivered to email or an app, and text messages sent from a business number.
- Fax over IP: Faxes sent as data using a standard called T.38, or converted to email.
- Presence and routing: Knowing who is available, and sending calls to the right person, team, or queue.
Because all of this runs as software on an IP network, a single system can handle it, instead of separate phone lines, fax lines, and voicemail boxes. That's also why IP telephony underpins cloud telephony and most modern business phone systems.
How Does IP Telephony Work?
IP telephony works by converting voice and other communications into digital data, splitting that data into packets, and sending the packets across an IP network to the other party, where they are reassembled. A call-control system sets up each call and decides where it goes.
A typical IP telephony system has four parts:
- Endpoints: IP desk phones, softphone apps on computers and mobiles, video devices, and adapters for older analogue phones and fax machines.
- Call control: The "brain" that registers devices, routes calls, and runs features such as menus, ring groups, and voicemail. It can be an IP PBX in the office or a platform run by a provider in the cloud.
- The IP network: The office LAN, Wi-Fi, and internet connection that carry the packets. Voice needs little bandwidth (roughly 100 kbps per call in each direction), but it does need a stable connection.
- Gateways and SIP trunks: The links to the public telephone network, so IP phones can call and receive calls from ordinary landlines and mobiles anywhere.
When you make a call, four things happen in quick succession:
- The call is requested: Your phone or app sends a signalling message to call control with the number you dialled.
- A route is chosen: Call control finds the other party, on the same system, another IP network, or the public phone network through a gateway, and starts the other phone ringing.
- Media flows as packets: Once answered, your voice is compressed by a codec and sent in small packets, usually every 20 milliseconds.
- The call is rebuilt and ended: The far end puts the packets back in order and plays them as sound. When either person hangs up, a final signalling message ends the call.
IP Telephony vs. VoIP
In everyday use, IP telephony and VoIP mean the same thing, and most people use them interchangeably. Strictly speaking, IP telephony is the whole category of telephone services built on IP, and VoIP is the technology that carries the voice part.
# | IP telephony | VoIP |
| Scope | All telephone services over IP: voice, video, fax, messaging, presence | Voice calls over IP |
| Typical use of the term | Enterprise IT, telecom, and system design | Everyday and business use |
| Includes | The phone system, devices, features, and connections to other networks | The method of carrying voice as packets |
| Example | A company phone system with IP phones, voicemail to email, fax over IP, and call queues | A single call from a mobile app to a landline |
So every VoIP call is part of IP telephony, but an IP telephony system usually does more than carry voice. If you're new to the topic, what VoIP is the simplest place to start.
IP Telephony vs. Traditional Telephony (PSTN)
The traditional public switched telephone network (PSTN) is circuit-switched: every call reserves a fixed path between two phones for its whole length, even during silence. IP telephony is packet-switched: calls share the network with other data and only use capacity when there is something to send.
# | IP telephony | Traditional telephony (PSTN) |
| Switching | Packet-switched; capacity shared | Circuit-switched; a path reserved per call |
| Network | Office LAN, internet, private IP networks | Copper lines and telephone exchanges |
| Devices | IP phones, apps, computers, several per number | Phones wired to one line |
| Location | Works anywhere with a connection | Tied to one address |
| Features | Routing, menus, recording, video, messaging, integrations | Basic; more needs an on-site PBX |
| Adding lines | Software change | New physical lines |
| Power cuts | Needs power for routers and phones; mobile apps keep working | Copper lines traditionally kept working |
| Future | Where networks are heading | Being retired in many countries |
The last row is what's pushing many businesses to act. The UK's analogue phone network is scheduled to close on 31 January 2027, and the PSTN switch-off guide explains what that means. In the US, carriers are retiring copper with the FCC's approval, so businesses on old lines are being moved to IP-based services.
Types of IP Telephony Systems
IP telephony can be deployed in four main ways. The difference is where call control lives and who runs it.
On-premise IP PBX The business buys an IP PBX (an appliance or software on a server) and runs it in its own building. It gives full control but brings upfront cost, maintenance, and the need for in-house skills. It's mostly found in large single-site organisations.
Hosted or cloud IP telephony A provider runs call control in its data centres, and staff connect through apps and IP phones. You pay per user per month and don't manage servers. Hosted VoIP is what most small and mid-sized businesses choose today.
Hybrid (IP PBX with SIP trunks) An existing on-site PBX keeps running, but the old phone lines are replaced by SIP trunks, which are virtual lines delivered over the internet. It's a common step for businesses with recent PBX hardware.
Unified communications Voice, video meetings, team chat, and messaging combined in one platform, usually delivered from the cloud.
The Protocols Behind IP Telephony
A handful of open standards make IP telephony work, and they're why equipment and services from different makers can talk to each other. You don't need to configure them, but the names come up in proposals and spec sheets.
SIP (Session Initiation Protocol) Sets up, changes, and ends calls and other sessions. It's the dominant standard in business phone systems and SIP trunking today.
H.323 An older family of standards for voice and video over IP, still found in some legacy video-conferencing and PBX equipment.
RTP and RTCP RTP carries the audio and video packets; RTCP reports on how well they're being delivered, which helps monitor call quality.
SRTP and TLS SRTP encrypts the audio itself, and TLS encrypts the signalling. Together they keep call content and call details private.
SDP (Session Description Protocol) Describes a session (which codecs, addresses, and ports to use) so both ends agree before media starts to flow.
T.38 The standard for sending faxes reliably over IP networks.
WebRTC Lets browsers make voice and video calls without plug-ins. It's how many browser-based calling tools work.
Key Features of an IP Telephony System
Because call control is software, IP telephony systems include features that used to need extra hardware and a specialist to set up.
- Auto attendant and IVR: An IVR menu greets callers and routes them without a receptionist.
- Call routing and ring groups: Business hours, ring groups, and call forwarding decide who answers, and where calls go after hours.
- One number, many devices: Staff take calls on a desk phone, laptop, or mobile app with the same number.
- Call recording: Call recording for training and compliance, where permitted.
- Voicemail to email: Messages arrive as audio and text.
- Call monitoring: Supervisors can listen, coach, or join calls with call monitoring.
- Integrations: Calls logged automatically in the CRM or helpdesk.
Benefits of IP Telephony for Business
The main benefits of IP telephony are lower running costs, flexibility about where people work, one system for several kinds of communication, and easier growth.
Lower costs No separate voice and data networks to maintain, fewer phone lines to rent, and international calls priced in cents per minute rather than carrier rates.
Work from anywhere The business number rings wherever staff are signed in, which suits remote and hybrid teams and branch offices.
One system for everything Voice, voicemail, video, fax, and messaging run on one platform with one directory, rather than separate services.
Easy to scale Adding a user or a number is a software change, not a new line and an engineer visit.
Better customer handling Menus, queues, and routing get callers to the right person, and CRM integration shows who is calling before you answer.
Future-proof As copper networks close, IP telephony is where phone service is heading, not an optional upgrade.
Challenges of IP Telephony
IP telephony's challenges mostly come from its reliance on the network and on power. None of them rules it out, but each needs a plan.
Network quality Calls are only as good as the connection. Congested Wi-Fi or a router without traffic prioritisation causes choppy audio. QoS for VoIP fixes most of it.
Power and internet outages IP phones and routers need power. A battery backup on key equipment and mobile apps that fall back to 4G/5G cover most outages.
Emergency calls Because IP phones can move, emergency services rely on the location registered with the provider. In the US, the FCC requires providers of interconnected VoIP to deliver 911 calls with location information, and staff need to keep registered addresses current.
Security Like any networked system, IP telephony needs encryption, strong passwords, two-factor login, and limits on international calling to prevent toll fraud. The VoIP security guide covers the risks.
Legacy equipment Alarm panels, lift phones, and older fax machines were built for analogue lines. Check each one before switching; some need an adapter or a dedicated replacement.
Who Manages IP Telephony?
Who manages IP telephony depends on how it's deployed. With an on-premise IP PBX, the business's IT team or a contractor handles installation, updates, security, and troubleshooting. With a hosted system, the provider runs the platform, and the business manages users, numbers, and call flows from a dashboard.
For most small and mid-sized businesses, the hosted model means no telecom specialist is needed. The practical jobs left in-house are adding and removing users, keeping emergency addresses current, and making sure the office network gives voice traffic priority.
Next step
Make the Move to IP Telephony
IP telephony is simply how phone service works now: calls and messages as data, on the networks you already use, managed by software rather than hardware. For most businesses the question isn't whether to move, but whether to run the system yourself or let a provider host it.
If you'd like to try a hosted option, every Calilio plan includes a free US, Canada, or UK local number and a monthly bundle of free minutes and SMS.
Frequently Asked Questions
Short answers to the questions people ask most about IP telephony.
What is IP telephony in simple terms?
IP telephony means running your phone services over an internet-style network instead of traditional phone lines. Calls, voicemail, video, fax, and messages travel as data, so they can be handled by software and used on phones, computers, and apps.
Is IP telephony the same as VoIP?
In everyday use, yes. Strictly, IP telephony is the broader category of phone services over IP, including video, fax, and messaging, while VoIP is the technology that carries voice calls.
What is an example of IP telephony?
A company phone system where staff use IP desk phones and mobile apps with one business number, get voicemail by email, and send faxes as email is IP telephony. So is a call centre that routes customers through menus and queues over the internet.
What is the difference between IP telephony and PSTN?
The PSTN reserves a dedicated circuit for each call over copper lines. IP telephony breaks calls into data packets and sends them over shared IP networks, which makes it cheaper to run, easier to expand, and usable from anywhere.
Does IP telephony need the internet?
It needs an IP network. Calls inside an office can run on the local network alone, but calls to outside numbers and remote staff need an internet connection or a private link to a provider.