1. Understand the two ways calls reach you
There are two underlying technologies, and most UK businesses now end up with a blend:
- Traditional telephone lines (ISDN or analogue) carry calls over physical copper wires into your building, terminating on a desk handset or a private branch exchange (PBX) box on the wall. BT switched off the UK's ISDN network in 2025, so any new install is now VoIP-based — but you may still have handsets that look traditional.
- VoIP (Voice over Internet Protocol) carries calls over your broadband connection instead. The same call can be answered on a VoIP desk handset that looks like a normal phone, on a computer using a softphone app, or on a mobile phone through an app. VoIP is cheaper per call, supports remote working, and is the default for new systems.
The key point: "VoIP" describes how the call travels, not what you hold in your hand. You can run VoIP on a traditional-looking desk phone or on a laptop — the choice is yours.
2. Choose your hardware: desk handsets, softphones, or both
Once the call arrives over VoIP, you decide what answers it. There are three options, and most businesses end up using a mix:
- VoIP desk handsets look and feel like a traditional office phone but plug into your network or connect over Wi-Fi. They suit people who sit at a desk all day, take a high volume of calls, and want a physical handset to pick up and put down.
- Softphones are software applications — the call comes in through your computer or mobile device. You dial, answer and transfer calls on screen. Because the audio runs through your computer, anyone using a softphone needs a headset with a microphone for clear sound and to keep their hands free while they talk and type. Speakerphones and laptop built-in microphones work in a pinch, but they pick up office noise and echo — a proper business headset makes a noticeable difference to call quality. If you are setting up softphones for your team, you can browse business headsets here to match the right headset to each user.
- Mobile apps let staff take their extension with them on a smartphone, which is ideal for field teams and anyone working from home. Calls show the office number, not the personal mobile number.
A common setup for a small office: desk handsets for reception and main lines, softphones for staff who move between desks or work remotely, and the mobile app for anyone on the road. The system should let you mix and match without buying a separate product for each.
3. Sort out your broadband first
VoIP calls are only as good as the internet connection behind them. If your broadband is slow, contended or drops out, your calls will suffer before anything else does — voice is the first thing to break. Before you commit to a system:
- Check your upload speed, not just download — uploading matters as much for calls.
- Run voice on a connection that is not maxed out by other traffic, or use Quality of Service (QoS) settings to prioritise voice traffic over downloads.
- Ask your supplier whether a dedicated voice connection or a backup 4G/5G failover is worth it for your setup.
4. Map out how calls should flow
Before comparing suppliers, sketch how a call should move through your business. Common questions:
- Should a call ring one person, a group, or everyone at once?
- If no one answers, where does it go — voicemail, a mobile, or another site?
- Do you need an auto-attendant ("press 1 for sales, 2 for accounts")?
- Do you need call recording for training or compliance?
- What happens out of hours — a different greeting, a diversion, or voicemail to email?
Any modern VoIP system can do all of the above. The differences are in how easy it is to change the setup yourself versus paying the supplier each time, and how cleanly it handles the flow when someone is working from home.
5. Check what happens when someone is out of the office
The biggest advantage of VoIP over a traditional handset is flexibility. A good system lets a member of staff log in from anywhere — home, a client site, or their mobile — and take their extension, their direct dial number and their call history with them. Ask the supplier to demonstrate a softphone on a laptop and the mobile app on a phone, not just a desk handset, so you can see the remote experience firsthand.
6. Can you keep your existing number?
Yes — UK regulations require that you can port your existing business phone number to a new provider, whether it is a geographic 01/02 number or a non-geographic 03 or 0800 number. Confirm porting in writing before you sign, and ask how long it takes and whether there is any downtime. Do not let anyone tell you the number "belongs" to the old supplier — it belongs to you.
7. Understand the contract and the exit
Telephone system contracts are where small businesses get caught. Before you sign:
- Check the minimum term and the notice period — auto-renewal clauses are common.
- Ask whether call bundles are included or billed per minute, and what the rates are for mobiles and international.
- Find out what happens to the hardware if you leave — do you own the handsets, or are they leased?
- Ask how you port numbers out if you switch providers later, and how long that takes.
8. The five questions to ask every supplier
- How many concurrent calls can this system handle, and is that enough for my peak?
- Can a non-technical person change call routing, voicemail and extensions without calling you?
- What is the total cost over the full contract term — hardware, lines, call charges, support and any setup?
- If my broadband drops, what is the backup — 4G failover, a second line, or nothing?
- Can I speak to two of your existing customers with a similar setup to mine?
Next step
Once you know what you need, comparing providers is much faster. Browse vetted UK telephone system suppliers in the telephone systems category, or read how to compare supplier quotes before you appoint one.