I've never had a power outage that outlasted the battery for the FIOS system. I do find I need to replace the batteries every 5 years or so.
If you have long outages, maybe you ought to keep a spare or two on hand. Or disconnect the battery except when you need to use the phone so that it doesn't drain. (I do buy my batteries at the local Battery Plus instead of from the phone company. Cheaper, although there are even cheaper options online. And I just leave the old one with them- no concerns about disposal.
What battery do you mean?
In the old days we had corded phones, that didn’t need any power or batteries. I always kept a corded phone for that reason. Now, that phone will go out along with a power failure, and the best they give us is a battery backup, which isn’t very convenient, BTW, to our layout/furniture situation.
I’m not talking about our remote phones, just those that did not use batteries.
I’m sure it will work out fine, but the point is we’ve lost true redundancy.
E.g., we have a back-up for our sump pump. No, it is NOT battery electric, it is pressure-based purely mechanics. IOW, it will work no matter how bad the situation.