OR FTTP may be powered at the head end with backup batteries and/or generators - but most Altnets use powered distribution cabinets.
I've actually just got off the phone with Gigaclear who have just built in my area - the distribution cabinet is connected to the same pole mounted transformer as my house, so will suffer the same power cuts as me. They think the batteries should last 3 hours (the fire I mentioned happened over 4 hours into a 7 hour power cut...).
That may be enough, as (depending on the power outage) we should get a mobile signal if we go half a mile down the road, and power cuts over 3 hours don't usually occur more than about once a year (short ones are annoyingly frequent though).
Also if we opt for Gigaclear, I may point my 4G directional router at a more distant mast to provide better resilience (even though the speed won't be as good).

In reply to a post by kitcat:

apart from wind damage to the actual fibres the FTTP is pretty robust to Power outages as long as you have UPS.

in 2025 when PSTN ends, there will still be a lot of customers on FTTC connections, which required powered cabinets. I read that Openreach do maintain batteries in FTTC cabs, and have in some circumstances installed generators to power cabinets with mains supply issues - but can they do that for 14 days in a Storm Arwen type scenario?

Depends on the draw, but most standby generators, unless they have enormous external tanks or are refilled regularly, definitely wouldn’t last a week without refuelling. They are usually sized to run up to a day or so. The generators used by telcos on exchanges or datacentres etc would have extended range tanks. Even they’ll need filling after a few days.

Planning for freak outage occurrences like Arwen is a like a hiding to nowhere. Where do you stop? Some bean counter will want justification for costs - you can see how this could all get cancelled or called into justification when the last storm fades from memory…

To be fair, in major disruptions such as the recent severe storms, the emergency services may not be in a position to respond to everyone, even if you can make a call.
However, the decision of what level of resilience is required, and what that means for the public, should be documented - and discussed.