BTwill replace traditional landline handsets as part of a huge digital changeover that experts say could hit the elderly hardest.
A new 'digital' phone will be favoured instead of a landline , which will force all homes online when the switchover happens in 2025.
The phone will be powered by electricity and will require an internet connection to make calls.
However, there are fears millions of elderly people who rely on traditional landline phones to communicate will be left isolated.
Pensioners groups have criticised the move by BT to force all homes online,ChronicleLivereports.
Jan Shortt, general secretary of the National Pensioners Convention, said: "If there is a power cut, this digital phone line will no longer work – and a potential lifeline for elderly people will be suddenly lost.
"Those needing to make an emergency call or raise an alarm via a health pendant could be left stranded – and unable to call anyone to ask for life-saving support."
Around six percent of households – roughly 1.5million homes – do not have access to the Internet, according to watchdog Ofcom.
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Those homeowners may need an engineer to visit their to get them set up and those with older phones could need to buy a new handset.
But experts fear millions more do not have a mobile phone or do not know how to use one properly.
"BT has no idea that many older people do not want a fancy smartphone or cannot afford one – and rely on their landline as a lifeline. It is wrong to discriminate against those – primarily the elderly – who are not wired up to the internet," Shortt added.
BT's switchover, branded 'Digital Voice', began two years ago and has so far converted two million landlines.
But with an end-of-2025 deadline for switching 29million home landlines, there are fears that the new system's flaws will be increasingly exposed.
Under guidelines laid down by Ofcom, BT must ensure customers can contact emergency services in a power cut that lasts more than an hour.
But how this can be achieved if you lose access from an internet phone line or do not have a mobile phone – or signal – is not clear.
In addition, charity Age UK is fearful the changeover could attract criminals claiming to be involved in the switch.
Caroline Abrahams says: 'We are concerned the changes could make many elderly feel more isolated than they do already. Steps must also be taken to ensure the vulnerable do not become victims of any digital voice scams."
Telecoms providers are obliged to make sure all households have access to the emergency services - although they can fulfil this by providing a mobile phone free of charge.
Openreach, which operates more of Britain’s wire and cable infrastructure, insists the protection of its most vulnerable customers is "an absolute priority".
A BT spokesperson said: "We’re proud of the support we offer to our vulnerable customers. Since 2018 we’ve been working with alarm and health pendant providers, and many now have a solution which works on the new digital line.
“If a customer is prone to power cuts and has no means of making a call in an emergency, then there are battery packs available which can power your equipment for over an hour.
"We’ll provide these for free for customers flagged as vulnerable on our system. If any customer has any concerns they should speak to us and we’ll find a solution which works for them.”