Experts are warning that older people could be "adversely impacted" if landline telephones are phased out in the next four years.
The switch to digital calls is being driven by the telecoms industry as part of a major shake-up.
Millions of landline customers would be forced to make calls through the internet or rely on mobile phones.
The upgrade would also reportedly impact other services that run on the existing telephone network, including alarm systems, phones in lifts, payment terminals and iconic red telephone boxes.
Telecoms giants are aiming for the switchover to be complete in 2025, according to the Daily Mail.
But experts have warned that millions of older and vulnerable households are not online.
Those who cannot get mobile phone or internet reception in remote and rural areas would also be negatively affected.
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Watchdog Ofcom believes about 6% of households in the UK – around 1.5 million homes – do not have access to the internet.
Caroline Abrahams, director of Age UK, said: "Landlines are a lifeline for millions of older customers, particularly those who are not online, don’t use a mobile, or live in a rural area with poor connectivity.
"The switchover to internet-based landlines should be straightforward for most customers, but there's a chance some could be adversely impacted."
Abrahams said that anyone without home broadband would need help with the transition to make sure they stay connected.
But she added: "Given that about half of older people over the age of 75 are not online, this could be a particular problem for our oldest citizens, especially if they lack the support of family or friends to help them."
Abrahams called on telecom providers to ensure customers are prepared for the changes and face as little disruption as possible.
"In addition, given the threat of fraud, they also need to take steps to prevent anyone who is in particularly vulnerable circumstances, for example because of dementia, from becoming victims of digital scams."
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