All land lines are now switching over to VOIP phones (voice over Internet) it’s not something you can avoid, and the switching off of the old land lines will open up extra bandwidth for speeding up internet access. If they are putting in a new cable it’s probably a fibre optic cable and therefore there will not be a wired phone connection any more.
If the broadband goes down then yes the phone will go down too, so if there was a power cut as an example your router would lose power then you’d have no phone, but that being said so many people have cordless phones in the house if they even still have a land line and so any power cut still would often leave you without any phone.
Technically speaking if there was a power cut the mobile phones shouldn’t go down unless there was some mass power cut covering a whole city and then the power would be cut to all the cell sites for relaying the mobile phone signals.
This sort of problem is pretty rare these days.
I’m with sky or should I say me and my wife are with sky for our mobile phones, and yes it rolls over all your unused data so you don’t lose any, if you ended your contract act you can spend your unused data on things
This is our current rolled over data
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So I use an iPhone 11 I got new in 2019 it works perfectly still and I should add it has 128gb of storage which is plenty for pictures and videos I take.
I think my total expenditure on my phone is less than £10 a month which I know is still quite a bit more than the £20 jocks phone has cost in the last year but I do use it a lot with all the various apps and other online stuff I use.