is it possible another mobile provider gives better coverage at your home you can check here
https://www.ofcom.org.uk/phones-and-broadband/coverage-and-speeds/ofcom-checker
Morning @Lesleyanne73
Thanks for coming to the community.
Can I just check, the device that is not working when you have a power cut is this a mobile phone or a Digital Voice phone with home broadband?
Speak soon.
Leanne.