BT Digital Voice Problems: What Goes Wrong (And What To Do)

BT Digital Voice is BT’s replacement for the copper landline: your calls move onto your broadband connection ahead of the PSTN switch-off on 31 January 2027. I want to be fair from the first paragraph — for a lot of households, it works fine. Good broadband, casual phone use, mobiles in every pocket as backup: Digital Voice is a reasonable answer, and it’s typically bundled with the broadband you’re already paying for.

But I’ve been in this industry since 2013, and the complaints about Digital Voice aren’t random. They cluster into a handful of predictable categories, and they land hardest on the people who actually depend on their landline. If that’s you — or your parents — this is the list worth reading before the migration letter gets actioned.

Problem 1: broadband down = phone down

This is the structural one, and no firmware update fixes it. With a copper landline, your phone and your broadband were separate things; one could fail without the other. With Digital Voice they are the same connection. Router reboots, line faults, an Openreach engineer working up the street, your ISP having a bad afternoon — any of it takes your landline with it.

For a household, that’s an annoyance. For a small business whose customers ring the landline, it’s invisible lost work: callers get dead air or endless ringing, and they don’t leave a note saying they tried.

Mitigation: BT’s app can take calls on your mobile during an outage, and honestly, that helps. But if the landline number genuinely matters, the more robust fix is to take the number off broadband entirely — more on that below.

Problem 2: the power cut behaviour

Old copper phones drew power from the exchange, which is why a basic corded phone worked in a blackout. Digital Voice doesn’t work that way. The router needs mains electricity, so a power cut silences the landline exactly when you might need it.

BT does provide battery back-up units — but the offer is aimed at customers classed as vulnerable or without mobile signal, because regulators require providers to protect access to 999 for people who depend on the landline. If you’re not in that category, the working assumption should be: power cut = no landline.

Mitigation: ask BT whether you qualify for a back-up unit, or buy a small UPS for the router. Or accept that your mobile is the power-cut phone — which is fine, as long as you’ve consciously decided that rather than discovering it mid-blackout.

Problem 3: dead extension sockets and handset quirks

After migration, your phone service comes out of the router — not the master socket in the hall. The extension sockets wired around the house over the years typically go dead. The phone in the bedroom, the one in the workshop: unless something is done about the wiring, they become ornaments.

Most modern handsets work fine plugged into the router, and DECT cordless systems with multiple handsets paired to one base solve the “phones in other rooms” problem neatly. But older or unusual kit — very old handsets, some fax machines, devices hanging off extension wiring — is where people get caught out.

Mitigation: count every device plugged into a phone socket anywhere in the building before migration day. A DECT multi-handset system covers most rooms. For anything analogue that must stay exactly as it is, see the alarms section below — the same answer applies.

Problem 4: alarms, telecare and lift lines

This is the category I’d genuinely call dangerous rather than annoying. Burglar alarm diallers, fire alarm signalling, telecare pendants for elderly relatives, lift emergency lines — an enormous amount of safety equipment was built to dial out over the copper network. Some of it works over a VoIP line. Some of it fails, and fails silently: the pendant looks normal, and nobody finds out until it’s pressed.

This was a major reason the switch-off deadline moved from December 2025 to January 2027 in the first place — the industry needed more time to migrate exactly these devices safely.

Mitigation: contact the alarm or telecare provider directly and ask, in writing, whether the device is supported on a digital line. Don’t rely on “it seems to work”. If the answer is no or maybe, a mobile-network analogue adapter — a box the device plugs into that carries calls over 4G — is the purpose-built fix. That’s exactly what ONSIM Landline Replacement is: works with analogue phones, alarms and lifts, no broadband involved.

Problem 5: call quality on congested broadband

VoIP call quality is only as good as the connection underneath it. On solid fibre, Digital Voice sounds excellent — often better than copper. On a long line, a busy household connection, or during the evening when everyone’s streaming, you can get the classic VoIP symptoms: robotic audio, words dropping out, one-way silence.

Mitigation: if your broadband is good, this probably never bothers you. If your broadband is marginal, no phone service running over that broadband will fix it — which is an argument for a phone service that doesn’t.

The pattern, summarised

Notice what every fix in the right-hand column has in common when the stakes are high: getting the phone service off the broadband.

When a mobile-network landline makes more sense

None of this means Digital Voice is a bad product. It means it’s the wrong product for a specific set of people: businesses that can’t miss calls, properties with weak or no broadband, households with analogue alarms or telecare, and anyone who wants the landline to keep working when the router doesn’t.

We’ve been running landline numbers over the mobile network at ONSIM since 2013 — no broadband, no app, no VoIP account — and the full comparison lives at our BT Digital Voice alternatives guide. The short version:

Virtual Landline — £5/month — your 01, 02 or 03 number rings on your existing mobile. Simplest possible way to keep your number after the switch-off, with nothing new to plug in.

Landline SIM — from £10/month — the landline number lives natively on a SIM in your phone, so outbound calls show the landline too. A normal mobile experience with a landline identity.

Landline Replacement — the answer to problems 3 and 4 above. Your existing analogue phone, alarm or lift line plugs into a small 4G box and carries on as before. £10/month plus a one-off £79 box, or £18/month with the box included.

All three ignore your broadband completely. When it goes down, when the power flickers, when the router is mid-update — your calls neither know nor care.

The switch-off itself isn’t optional, and I’ve written before about why BT is really doing it. But which replacement you end up on absolutely is optional. Read the migration letter, look at the list above, and decide which column you’re in — before January 2027 decides for you.

Luke Faragher is the founder of ONSIM, which has provided SIM-native virtual landlines to UK businesses since 2013. ONSIM is an independent mobile network and is not affiliated with BT Group or Openreach.