Thousands of people have signed a petition urging ministers and the telecoms industry to delay the switchover to a digital system.
BT, Virgin and others say the new technology will be easier to maintain and perform better.
But campaigners say 999 calls will be impossible if there is a power cut as the new “Digital Voice” phones only work with a household electricity supply.
They also rely on broadband, but 1.5 million homes are not online.
Dennis Reed, director of the campaign group for the over-60s, warned: “It is no longer good enough for the Government to leave this major infrastructure programme to the Telecom Industry.
“BT and the other telecom giants are driven by commercial considerations and are doing the bare minimum to support vulnerable customers.”
“The companies do not have comprehensive lists of vulnerable customers and we have already had cases of people living with dementia having their landlines cut off.”
Mr Reed said the Post Office scandal should serve as a warning to the government.
“Politicians must learn the lessons of the Post Office scandal and act now to prevent potential hazards and tragedies,” he added.
“Do we have to wait for the first fatality caused by a personal alarm system not working, or an emergency call not getting through because of an internet outage or power cut?
“I hope our petition will force the Government to take a more hands on approach, after all they agreed this unrealistic timetable with the industry in the first place.”
The petition, which calls for a delay for at least five years in the programme, is also supported by the Digital Poverty Alliance.
More than 6,500 people have already backed it.
Culture and Media Secretary Michelle Donelan has already called on the telecom industry to protect vulnerable customers in the switchover, particularly those relying on personal alarm systems which may not work with the digital system.
Campaign groups say all older and vulnerable customers need to be able to rely on a failsafe emergency call system, which we have at the moment with traditional landlines.
With Digital Voice (or VoIP), if there is a power cut or internet outage, they claim the new telephones will not work.
A BT spokesperson said: “Silver Voices was a founding member of our Digital Voice Advisory Group and is fully aware of the need to switch off the old analogue network by the end of 2025 because it is increasingly fragile and unreliable.”
“This isn’t a choice by industry, but a technological necessity, and together with Government, Local Authorities and telecare providers, we’re committed to working together to identify and protect telecare users and those with additional needs before they switch.”
Matthew Evans, Director for Markets at techUK, said: “The transition to digital telephone lines is essential as we move towards fit for the future digital networks. The current PSTN system is over 40 years old and increasingly unreliable – with faults rising 20 per cent in 2023.
“Many other countries have gone through this migration and many millions of households in the UK have already completed it. We will continue to work with the telecoms sector as well as other important parties to deliver a safe and sustainable switch”.
Silver Voices petition and the current level of support can be accessed on petition.parliament.uk/petitions/653201
