“How many phone lines does my business need?” is one of the first questions owners ask when they replace an old phone system. With traditional landlines, the answer meant counting physical lines. With a modern VoIP system, the question changes. You think about users and simultaneous calls instead. This guide explains how to size a system for your team, with worked cost examples for 8, 15 and 30 users.
Why “lines” mean something different with VoIP
With a traditional landline or ISDN set-up, each line could carry one call at a time. If you had four lines and four calls were in progress, the fifth caller got an engaged tone. Adding capacity meant ordering more lines and often waiting for an engineer.
With a hosted VoIP phone system, calls travel over your internet connection. There’s no physical line per call. Instead, sizing comes down to two things:
- Users: the people who need their own extension to make and take calls.
- Concurrent calls: the number of calls happening at the same time across the business.
Our hosted PBX page explains how a cloud phone system works in more detail.
Step 1: Count your users
A user is anyone who needs to make or receive calls on their own extension. That usually includes:
- Reception and front-desk staff.
- Sales, account management or customer service staff.
- Managers and directors who take client calls.
- Staff working from home or on the road who need the business number on their mobile.
It’s worth thinking beyond who has a desk phone today. Under an older system, some staff may share a phone or use personal mobiles. Giving them a proper extension, even if it’s only on the mobile app, means calls are answered and reported in one place, and clients have a business number rather than a personal one.
Shared areas and devices
Some extensions don’t belong to one person, such as a phone in a meeting room, a warehouse or a staff kitchen. Ask your provider how these are handled and priced, as it can vary.
Step 2: Estimate how many calls happen at once
Concurrent calls are the number of conversations in progress at the same moment, inbound and outbound combined. They matter for two reasons:
- Your internet connection needs enough capacity to carry them clearly.
- Your call routing should handle busy moments, such as queues or overflow, instead of turning callers away.
The number of concurrent calls depends heavily on the type of business. In an accountancy practice, only a portion of staff may be on the phone at any one time. In a recruitment agency or a busy lettings office, a much larger share of the team may be on calls at once during peak periods.
The simplest way to estimate is to look at your busiest times. How many people are on the phone at 9.30am on a Monday, or at lunchtime when callers are on their breaks? If you have call records from your current system, they’ll give you a much better picture than guesswork.
Step 3: Check your internet connection
Because every call uses your internet connection, it’s important the connection is reliable and has enough capacity for your busiest moments, alongside everything else your business does online. Each call uses a fairly small amount of bandwidth, but quality depends on a stable connection as well as speed.
If you’re unsure, a provider should be able to look at your current connection and router and tell you whether they’re suitable before you switch.
Step 4: Plan your call flows
Once you know your users and peak call levels, plan what happens to each call:
- Ring groups: which teams should ring when someone calls the main number?
- Queues: should callers wait in a queue with a message if everyone is busy?
- Menus: would “press 1 for sales, 2 for accounts” save reception time?
- Out-of-hours rules: what happens when the office is closed?
Good call flows mean fewer missed calls without needing more staff.
What does it cost? Worked examples
Our business phone system is priced per user, from £13.49 per user per month on yearly billing, or £14.99 per user per month on monthly billing. Features such as ring groups, queues, call recording, reporting, voicemail to email and the desktop and mobile apps are included. Number transfer is free.
| Team size | Yearly billing (from £13.49/user) | Monthly billing (from £14.99/user) |
|---|---|---|
| 8 users | from £107.92 per month | from £119.92 per month |
| 15 users | from £202.35 per month | from £224.85 per month |
| 30 users | from £404.70 per month | from £449.70 per month |
These figures are for user licences. Handsets, call bundles and any extra numbers depend on your needs, so they’re confirmed in your quote. Our guide to business phone system costs in the UK covers the other items to budget for.
Example: an 8-user practice
A small professional practice with a receptionist and seven staff might have most people taking occasional client calls, with a ring group on the main number and voicemail to email out of hours.
Example: a 15-user office across two sites
A business with two offices could put both on one system, transfer calls between sites by extension and keep each office’s local number.
Example: a 30-user sales or recruitment team
A call-heavy team would typically use queues, call reporting and recording, with consultants using desktop apps and mobile apps when out of the office.
How many phone lines does my business need? Avoid these sizing mistakes
- Only counting desk phones: staff on mobiles still need an extension if they take business calls.
- Ignoring peak times: plan for your busiest hour, not an average day.
- Forgetting growth: adding users to a hosted system is easy, but budget for the team you expect to have.
- Skipping a connection check: call quality depends on your internet connection.
The switch-off deadline
If your business still uses analogue or ISDN lines, remember that the UK’s traditional phone network is being switched off, with a deadline of 31 January 2027. Sizing your new system now means you can plan the move calmly.
If you’d like help working out the right size for your business, call 0333 307 3500 or request a quote.
Frequently asked questions
Do I need one phone line per member of staff?
Not with VoIP. Each person who takes calls has an extension, and the system handles multiple simultaneous calls over your internet connection rather than physical lines.
Can I add users later?
Yes. Users can be added as your team grows, without new lines being installed.
Can I keep my existing number?
In most cases, yes, and number transfer is free.
Is yearly or monthly billing better?
Yearly billing has the lower price, from £13.49 per user per month. Monthly billing, from £14.99 per user per month, gives more flexibility.