Choosing between VoIP vs a traditional phone system is one of the first decisions a growing or modernising business has to make — and with the PSTN and ISDN switch-off underway, it’s a decision most UK businesses can no longer put off. Here’s a clear, practical comparison to help you decide.

What Is a Traditional Phone System?

A traditional phone system relies on PSTN (Public Switched Telephone Network) or ISDN lines — physical copper cabling connecting your office to the local telephone exchange. Calls are carried as analogue or digital signals over this dedicated line, usually through an on-site PBX (Private Branch Exchange) unit.

What Is VoIP?

VoIP (Voice over Internet Protocol) carries calls as data over your existing internet connection instead of a separate phone line. Rather than a box in a server room, your phone system lives in the cloud, and calls can be made from desk phones, mobile apps, or a browser.

VoIP vs Traditional Phone System: Side-by-Side Comparison

Factor	Traditional Phone System	VoIP / Cloud Phone System
Setup	Physical lines, on-site PBX hardware	Software-based, works over existing internet
Cost	Line rental, call charges, hardware maintenance	Predictable monthly subscription, lower call costs
Scalability	Requires engineer visits to add lines	Add users in minutes via self-service portal
Remote working	Limited to the physical office line	Calls follow staff on mobile, desktop, or browser
Features	Basic call handling only	Video, messaging, CRM integration, analytics, contact centre tools
Reliability	Single point of failure at the office	Calls can reroute automatically if internet drops
Future-proofing	Being phased out by the PSTN switch-off	Built on the technology replacing PSTN/ISDN

Cost: Which Is Actually Cheaper?

Traditional phone systems come with ongoing costs that are easy to underestimate: line rental per number, call charges (especially for mobile or international calls), and maintenance contracts for ageing hardware. VoIP typically consolidates these into one predictable monthly cost per user, often with call bundles included. For a full cost breakdown, see our guide on business phone system costs in the UK.

Features: What You Gain by Switching

Moving from a traditional phone system to VoIP isn’t just a like-for-like swap — it typically unlocks features that simply aren’t possible with copper-based lines:

• HD voice calling with call recording and analytics
• Video meetings and screen sharing in the same app
• Presence status, so staff can see who’s available before calling
• CRM integration for click-to-dial and automatic call logging
• Contact centre tools like call queuing and wallboards for larger teams

Reliability: Is VoIP as Dependable as a Landline?

This is one of the most common concerns businesses raise, and it’s a fair question. Traditional lines feel reliable because they’re familiar, but they’re also a single point of failure — if the office line goes down, so does the phone system. A well-managed VoIP platform, by contrast, is hosted across multiple data centres and can automatically reroute calls to mobiles or another site if there’s an issue, making it more resilient in practice, not less.

The PSTN Switch-Off Changes the Comparison

This decision is no longer purely about preference. Openreach is retiring the PSTN and ISDN network exchange by exchange, meaning traditional phone lines will eventually stop working altogether regardless of which provider a business uses. Read our full breakdown in PSTN Switch-Off Explained to see how this affects your migration timeline.

Which Should Your Business Choose?

A traditional phone system may still make sense for a very small number of businesses with no future need for flexibility, remote access, or modern features. For almost everyone else, VoIP offers lower costs, more features, and — critically — a system that will still be supported once PSTN and ISDN lines are switched off for good.

How Curo IP Makes the Switch Simple

Curo IP is a fully managed cloud phone system built specifically to replace ageing PSTN and ISDN setups without disruption. We handle number porting, hardware, and staff onboarding, with most UK businesses fully migrated within 5 working days. Get a free quote to see what switching would look like for your business.