A telephone exchange is the infrastructure that connects phone calls between callers. It routes your call from your phone to the recipient’s phone – whether they are in the next office or on another continent. In 2026, the UK’s telephone exchange network is undergoing its biggest transformation in over a century.
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BT’s Openreach is switching off the entire analogue exchange network by January 2027, replacing it with an all-digital, all-IP system. This guide explains how telephone exchanges work, the different types, and what the switch-off means for UK businesses that rely on traditional phone lines.
- Call accounting declining - 70% of businesses no longer need separate billing systems thanks to cloud phone analytics
- Still needed for billing - Service companies charging clients for calls require detailed call accounting systems
- Built-in analytics sufficient - Modern phone systems include reporting that replaces 80% of traditional call accounting needs
- £500-5,000 annual costs - Dedicated call accounting software prices vary dramatically based on call volume
- Real-time reporting essential - Look for systems providing instant call analytics rather than end-of-month reports
What Is a Telephone Exchange?
A telephone exchange is a switching system that connects calls between subscribers. It receives a call from one phone line and routes it to the correct destination based on the dialled number.
When you make a phone call, your voice signal travels from your phone to your local exchange. The exchange reads the number you dialled and determines where to route the call. If the recipient is on the same exchange, the connection is made locally. If not, your call is passed through a hierarchy of exchanges until it reaches the destination.
The UK’s telephone exchange network is managed by Openreach (BT’s network division) and consists of approximately 5,600 local exchanges serving homes and businesses across the country. These exchanges have traditionally used analogue and digital circuit-switching technology – but that is changing.
Types of Telephone Exchange
The four types of telephone exchange are manual switchboards (obsolete), electromechanical (Strowger, obsolete), digital circuit-switched (current PSTN, being retired), and IP-based packet-switched (the replacement).
| Type | Era | How It Works | Status in 2026 |
|---|---|---|---|
| Manual switchboard | 1878-1960s | Human operators connected calls using patch cords | Obsolete – museum pieces |
| Electromechanical (Strowger) | 1890s-1990s | Rotating mechanical switches selected circuits based on pulse dialling | Fully decommissioned in the UK |
| Digital circuit-switched | 1980s-present | Electronic switches establish dedicated circuits for each call (System X, AXE-10) | Being retired – PSTN switch-off by Jan 2027 |
| IP packet-switched | 2000s-present | Voice converted to data packets, routed over broadband networks | The replacement – all new connections are IP |
The UK’s current PSTN runs on digital circuit-switched exchanges (primarily System X, developed by GEC Plessey Telecommunications, and Ericsson’s AXE-10). These exchanges dedicate a physical circuit to each call for its entire duration – reliable but inefficient, as the circuit is reserved even during silence.
IP exchanges break voice into data packets that share bandwidth with other traffic. This is fundamentally more efficient and enables the advanced features (video, messaging, presence, AI) that modern VoIP systems provide.
Public vs Private Exchanges
Public exchanges (the PSTN) connect calls across the national phone network. Private exchanges (PBX systems) handle internal calls within a business and connect to the public network for external calls.
| Exchange Type | Operated By | Purpose | Example |
|---|---|---|---|
| Public (PSTN/PSTN exchange) | BT Openreach / network operators | Routes calls between any two phone numbers in the UK and internationally | Your local BT exchange building |
| Private (PBX) | Individual businesses | Routes internal calls between extensions, connects to public network for external calls | Office phone system with extension numbers |
| Virtual/Cloud PBX | VoIP providers | Same function as PBX but hosted in provider’s data centres, not your office | RingCentral, bOnline, Dialpad |
For businesses, the private exchange (PBX) is what you interact with daily – it is your business switchboard. The public exchange is the national network infrastructure that connects your PBX to the outside world. When someone outside your company calls your main number, the public exchange routes the call to your PBX, and your PBX then routes it to the correct extension.
With cloud VoIP, the private exchange function moves to the provider’s servers. Your business no longer needs physical PBX hardware – the provider handles all call routing, and your phones connect over broadband. Read our guide to PBX phone systems for a detailed comparison.
The UK PSTN Switch-Off Explained
BT Openreach is retiring the entire UK analogue telephone exchange network by January 2027. All voice calls will move to IP-based connections running over broadband.
The PSTN switch-off is the biggest change to UK telecommunications since the network was built. Here is the timeline:
- September 2023 – Openreach stopped selling new analogue (PSTN) and ISDN lines
- 2023-2026 – Exchange-by-exchange migration to all-IP infrastructure
- January 2027 – Target date for complete PSTN shutdown across the UK
What stops working:
- Traditional analogue phone lines
- ISDN connections (ISDN2, ISDN30)
- Fax machines connected to analogue lines
- Burglar alarms, CCTV, and lift phones that use PSTN lines
- Chip-and-PIN card machines connected via phone lines
- Any on-premise PBX system that connects to analogue or ISDN lines
What replaces it: All voice services move to Digital Voice – calls carried as data over your broadband connection. For businesses, this means migrating to VoIP (cloud VoIP providers) or SIP trunking if you want to keep existing IP-PBX hardware.
What Should Your Business Do?
If your business phone system connects to analogue or ISDN lines, migrate to a cloud VoIP provider before January 2027. Most providers complete the switch in 1-2 weeks including number porting.
The right migration path depends on your current setup:
| Your Current Setup | Migration Path | Cost | Timeline |
|---|---|---|---|
| Analogue phone lines (no PBX) | Switch to cloud VoIP provider (bOnline, RingCentral) | From £7/user/month | 1-2 weeks |
| ISDN with on-premise PBX | Replace PBX with cloud VoIP, OR keep PBX and add SIP trunks | Cloud: £7-25/user. SIP: £4-13/trunk | 2-4 weeks |
| IP-PBX (already on broadband) | No action needed – already IP-based. Consider SIP trunking for cost savings | SIP: £4-13/trunk | N/A |
| Cloud VoIP (already hosted) | No action needed – completely unaffected by the switch-off | N/A | N/A |
For most small and mid-sized businesses, the simplest option is moving to a cloud VoIP provider. Your existing phone numbers can be ported across, and the provider handles all the technical infrastructure. Compare options in our guide to the best business phone systems.
Our Summary
The era of analogue telephone exchanges is ending. UK businesses should treat the January 2027 deadline as a modernisation opportunity, not just a forced migration.
Telephone exchanges have been the invisible backbone of UK communications for over 140 years. The shift to IP is not just a like-for-like replacement – it unlocks capabilities that circuit-switched networks could never provide: video conferencing, team messaging, AI transcription, CRM integration, and the ability to work from anywhere on any device.
For businesses still on traditional phone lines, the migration is straightforward and typically saves money. A 10-person office paying £300-£500/month in ISDN line rental and PBX maintenance can switch to cloud VoIP for £70-£250/month – with more features included.
Start comparing providers: RingCentral, bOnline, Dialpad, Vonage, GoTo Connect, Google Voice. For businesses that want to keep existing PBX hardware, see our guide to SIP trunk providers.
