The BT VoIP Phone Switch | What UK Businesses Need to Know

What Is the BT VoIP Switch?

BT is undertaking one of the most significant infrastructure changes in UK telecoms history: the retirement of the Public Switched Telephone Network (PSTN). The PSTN is the copper wire network that has carried telephone calls in the UK for over a century. BT's Openreach division is systematically decommissioning this network and replacing it with a fibre and IP-based infrastructure.

The practical consequence for businesses is that any phone line connected to the old copper network — traditional business lines, ISDN2, ISDN30, ADSL broadband over copper — will eventually cease to work. All voice services must migrate to VoIP (Voice over IP), which transmits calls as data over an internet connection.

The PSTN Switch-Off Timeline

The BT PSTN switch-off has proceeded in stages by exchange area since pilot programmes began in 2021. Full switch-off has been completed in many exchange areas across the UK. The process is continuing through 2025 and 2026 as Openreach works through the remaining exchanges.

BT and Openreach communicate directly with customers in areas approaching switch-off. However, businesses should not wait for a letter before acting — many have been caught by compressed timescales when they assumed they had more time.

What Has Already Changed

• ISDN2 and ISDN30 services have been ceased in the majority of UK exchange areas
• New PSTN/ISDN line orders are no longer accepted in most areas
• Broadband services over legacy copper are being phased out in areas covered by full-fibre infrastructure

What the VoIP Switch Means for Your Business

Phone Calls Will Travel Over Your Internet Connection

Once migrated to VoIP, all calls are transmitted as data over your broadband or leased line connection. This means call quality depends on the quality and reliability of your internet connection. A stable, business-grade connection — ideally a leased line or FTTP — is strongly recommended.

You May Need New Hardware

Traditional desk phones designed for PSTN or ISDN lines will not work with a VoIP system. Businesses need either IP phones (designed for VoIP), softphones (apps on laptops or mobiles), or analogue telephone adaptors (ATAs) if they want to continue using existing handsets.

Alarm and Fax Lines Need Special Attention

Many businesses have secondary lines for fax machines, door entry systems, lift alarms, and CCTV that are often overlooked during telephony reviews. These must all be migrated or replaced. Fax can typically be replaced with an internet fax service. Alarm and lift lines may require consultation with the relevant installer or manufacturer.

Call Quality Will Depend on Your Broadband

A single VoIP call uses approximately 80–100 kbps of bandwidth. For a business with 10 concurrent calls, this is around 1 Mbps — manageable for most business broadband connections. However, quality issues arise when VoIP traffic competes with other business data on an unmanaged connection. Quality of Service (QoS) settings on your router, or a dedicated voice VLAN, are important considerations.

How to Prepare for the BT VoIP Switch

1. Audit Your Existing Lines

Before anything else, establish exactly what lines you have, what they are used for, and which services depend on them. Include PSTN lines, ISDN channels, alarm lines, lift lines, and fax lines. Many businesses discover lines they had forgotten about during this process.

2. Review Your Broadband Connectivity

VoIP requires a reliable, business-grade internet connection. Assess whether your current broadband is adequate. If you have multiple concurrent callers, a consumer-grade FTTP connection may introduce quality issues. A leased line provides a dedicated, uncontended connection that guarantees bandwidth and latency — well suited to businesses making VoIP the primary communications medium.

3. Choose a VoIP Provider

Rather than defaulting to whatever BT offers as your migration solution, take the opportunity to evaluate the market. Independent VoIP providers often deliver better features and lower costs than BT's own hosted products. Key providers in the UK market include Gamma Horizon, 3CX, RingCentral, 8x8, and Microsoft Teams Phone.

4. Plan Number Portability

Your existing BT phone numbers can be ported to your new VoIP provider. Number porting typically takes 5–10 working days. Coordinate this carefully to avoid a gap in service — your new VoIP system should be tested and live before the old lines are ceased.

5. Replace or Adapt Hardware

Arrange IP handsets, softphone licences, or ATAs as needed. If your team works from multiple locations or remotely, a softphone app may be all that is required. For businesses that rely on physical handsets, a like-for-like IP phone replacement is the straightforward path.

The Opportunity Within the Obligation

The PSTN switch-off is an obligation, but it is also an opportunity. Many businesses that have been running ageing on-premise PBX systems will find that migrating to a hosted VoIP platform reduces their IT maintenance burden, lowers costs, and adds features — call recording, analytics, mobile working, CRM integration — that their old system never provided.

AMVIA works with UK businesses to plan and execute smooth PSTN migrations, handling the connectivity review, VoIP provider selection, number porting, and hardware provisioning as a managed process.