A fully loaded Legend carrier (not including expansion carriers) is rated by Avaya to consume about 160 watts of power, with a peak current of 5.4 amps. (Note that 5.4 amps at 120v is more than 600 watts, so that peak current is not at all typical.)
In selecting a UPS, figure out how many minutes of power you want, and select a unit that can handle the watt load. Most UPS specs will show how many minutes they will run at a stated watt load.
For example, check at
for some sample units from American Power Conversion (APC). This page lists several units, and you can check out the run-times.
Since you haven't mentioned long-term blackouts, you probably only need a few minutes of protection, though seems like 30 minutes is a good idea - to allow for phone use during an emergency.
There are many good brands out there. Tripp-Lite, APC, Best come to mind. You can use the UPS units sold for computer usage, and can get some good bargains at the larger computer stores. Look for brands that offer a guarantee of equipment protection (like $50k coverage if there's a lightning strike).
In the long run, a UPS requires maintenance. Batteries should be tested annually, and replaced perhaps every 5 years. Your customer will need an ongoing process if they are
counting on perfect uptime. That's beyond the scope of this article. All we're trying to do here is improve things from intolerable to 99% reliable.