I still think it a stupid idea, currently phones are powered from the exchange, and the exchange is required to keep a battery backup that lasts for days.
They will provide battery backups, but only to selected "vulnerable" customers, those backups only last for an hour, and from what I can see the customer will be responsible for replacing it when it eventually wears out. They will say to use a mobile but many people can't, and many mobile phone masts either have no or inadequate battery backups.
They're still trying to force people into it though, you can't switch to BT without taking one of their digital "lines". I tried last month, and despite having multiple vulnerable people in the house they wouldn't let me.