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ONSIM vs BT Digital Voice: two ways to keep your landline
This one isn't a like-for-like product shootout - it's a comparison of approaches. BT Digital Voice moves your home phone onto your BT broadband (VoIP). ONSIM moves your landline number onto the mobile network instead - as a divert to your mobile, a SIM that carries the number, or a box your analogue phone plugs into. Both survive the January 2027 PSTN switch-off; they fail (and shine) in different places.
For the full deep-dive on your options, see our BT Digital Voice alternatives guide.
Side-by-side
BT Digital Voice details checked on bt.com, July 2026. ONSIM prices ex VAT. Verify live before deciding.
| ONSIM | BT Digital Voice | |
|---|---|---|
| What it is | Landline over the mobile network (divert, SIM, or ATA box) | Home phone as VoIP over BT broadband |
| Requires broadband | No | Yes (BT broadband + Smart Hub) |
| Standalone product | Yes - works with any (or no) broadband/mobile provider | No - bundled with BT broadband plans |
| Price | £5/mo (divert) · £10/user/mo (SIM) · £10-£18/mo (Replacement box) | No extra cost on top of your BT plan (per bt.com) |
| Works in a power cut / broadband outage | Yes - mobile network (phone needs charge/signal) | No - including 999; BT advises keeping a mobile charged |
| Landline rings your mobile anywhere | Yes - that's the core product | Partially - call diversion to a mobile is a feature |
| Outbound calls show landline number from a mobile | Yes (Landline SIM, native) | No - calls from the home handset(s) |
| Keep existing analogue handset | Yes (Landline Replacement box) | Yes (plug into Smart Hub or free adaptor) |
| Telecare / alarms / lifts | Supported via Landline Replacement (check with alarm provider) | May not be supported - BT advises checking first |
| Scam-call screening | No equivalent feature | Yes - Call Protect included free |
| Keep your number | Yes - porting typically 10-14 working days, fee covered | Yes - number unaffected by the switch |
| Best suited to | Businesses and households wanting independence from broadband/BT | BT broadband households keeping a home phone at home |
When ONSIM is the better choice
- You don't have - or are dropping - BT broadband, and want the landline to be independent of any broadband
- You want the landline to ring on your mobile wherever you are, not at a socket at home (Virtual Landline, £5/mo)
- You're a business and need outbound calls from mobiles to display the landline number (Landline SIM, £10/user/mo)
- You have analogue phones, alarms or lifts that must keep working without broadband (Landline Replacement)
- You want calls to keep working during broadband outages and power cuts (via your charged mobile)
When BT Digital Voice is the better choice
- You're staying on BT broadband anyway - Digital Voice adds no extra cost to your plan
- You mainly use the home phone at home, from a handset, and want the simplest possible switch (plug into the Smart Hub)
- You value the free extras - Call Protect AI scam screening, caller display, voicemail, HD calling on BT digital handsets
- You'd rather have one bill and one provider for broadband and phone
More about Digital Voice at bt.com.
Frequently Asked Questions
Common questions before choosing between the two approaches.
What is BT Digital Voice, in plain terms?
BT Digital Voice is BT's replacement for the analogue home phone: your calls run as VoIP over your BT broadband connection, with the phone plugged into the back of the BT Smart Hub (or connected via a free adaptor). It exists because the copper PSTN network is being switched off by the end of January 2027. It's a consumer home-phone service bundled with BT broadband - not a standalone product you can buy separately, and not the same category as ONSIM.
How is ONSIM different from BT Digital Voice?
Same goal - keeping a landline number alive after the PSTN switch-off - but opposite infrastructure. BT Digital Voice routes calls over your home broadband; ONSIM routes them over the mobile network. That gives you three ONSIM shapes: a £5/month Virtual Landline that rings your mobile, a £10/user/month Landline SIM that puts the number on a SIM/eSIM, and a Landline Replacement box that lets your existing analogue phone keep working with no broadband at all. Our full guide is at BT Digital Voice alternatives.
How does pricing compare?
BT says switching to Digital Voice costs nothing extra - you keep paying your existing home-phone/broadband plan, which is genuinely good if you're staying with BT broadband anyway. ONSIM is priced standalone: £5/month + VAT (Virtual Landline), £10/user/month + VAT (Landline SIM, unlimited UK calls and SMS under fair use), or Landline Replacement at £10/month + £79 box or £18/month with the box included. The real comparison is: bundled with BT broadband vs independent of any broadband.
What happens in a power cut or broadband outage?
This is the biggest practical difference. BT Digital Voice stops working in a power cut or broadband outage - including 999 calls - and BT's own guidance is to keep a charged mobile handy (battery back-up units are offered to vulnerable customers). ONSIM services run on the mobile network: a Virtual Landline or Landline SIM keeps working as long as your mobile has charge and signal, regardless of your broadband.
When is BT Digital Voice the better choice?
If you're a BT broadband customer, happy with BT, and mainly use your home phone at home - stay put. Digital Voice costs nothing extra on top of your plan, the switch is usually just plugging your phone into the Smart Hub, you keep your number, and you get free features like Call Protect scam-screening, caller display and voicemail. For a straightforward BT household, it's the path of least resistance and there's no need to overthink it.
When is ONSIM the better choice?
Pick ONSIM if you don't have (or don't want) broadband from BT; if you want the landline to ring on your mobile wherever you are rather than at a socket in the hallway; if you're a business that wants outbound calls showing the landline number on mobiles (Landline SIM); or if you have analogue phones, alarms or lifts that must keep working without broadband (Landline Replacement). Also if you're leaving BT broadband - Digital Voice is tied to it; ONSIM isn't tied to any broadband.
Can I move my BT landline number to ONSIM?
Yes. UK number portability is mandatory, and moving a BT number to ONSIM typically takes 10-14 working days - the port-in fee is covered. This is a common route for people who want to drop the BT line rental or broadband bundle but keep a decades-old family or business number. See keeping your landline number.
What about telecare alarms, burglar alarms and lifts?
BT itself cautions that telecare and burglar alarms may not be supported on Digital Voice and tells customers to check with their alarm provider before switching. ONSIM's Landline Replacement box is designed for exactly this: analogue phones, alarms and lifts plug in and continue working over the mobile network. If you have such equipment, check compatibility with your alarm provider whichever route you take.