Just to add some support to Barry and others:
All UK phone lines are being switched over from the current service (whereby there's power made available through the phone socket to route calls over the exchange network) to VOIP ('voice over internet protocol'), which BT calls 'Digital Voice'.
Under the new service, everyone will have a router in their home (even if they're not internet users), new-style phones connecting by wi-fi to them. Calls will then be passed over the internet, rather than through the exchange network, in a similar way to how Skype calls are made now.
It will no longer be possible to connect a basic phone to a wall socket, as there will be no power to the line to get it to work and no exchange network for it to connect to.
See here for BT's confirmation that people won't be able to use landline phones during a power cut in future:
https://www.bt.com/help/landline/digital-voice--will-my-service-work-in-a-power-cut-
Barry's suggestion (of ensuring that one has a mobile phone available for such suggestions) is clearly a sound one but it still doesn't address the problem of what will happen if a major power outage knock mobile phone masts out of service too.