Your question is a little light on detail.
So I'm going to suggest you take a step back for a moment and consider what might seem to be too obvious.
Q. When the power is out, are all or only some employees able to work?
A. Honestly, I think the answer is quite obvious. Most businesses don't have large UPS's for their computers and machinery that allow their staff to keep working.
Yet, we're so reliant on electricity there is not much you can do without it.
Since most businesses can't do much without power to computers and machinery I would think that a phone is of little use except in an "emergency" situation. Even a call centre can't do much without their computers running. You need to weigh this up against the cost of putting in a bigger UPS and providing UPS power to the phones via PoE.
Nowadays, most employees will have a cell phone which they can use in case of emergency (911 call) if the office phone system is not working.
Unless you're hosting web sites or other things I suggest you're wasting your time and money even contemplating making the phones still work in a power outage. The employees won't be able to do much anyway.
However, a UPS on servers can be very beneficial. Not having to reboot the server is a good thing if it can survive the black out period. And if it can't survive the blackout period, having the UPS signal the server that it's going to loose power soon so that the server can do an orderly shutdown is also a very good thing.
So I'd suggest you ensure your servers can keep running for around 40+ minutes off battery. I'd not bother with individual phones but if you want you could power those from PoE.