I once played my digital piano for several hours on a 450 watt (?) UPS during a blackout. That was with headphones, not the speakers. So a UPS can indeed last a long time powering a low wattage device.

But rather than trying to provide backup power for VOIP, I'd just go for a cellphone. And have a separate rechargeable power bank. And use text messages rather than voice calls. For one, text messages take a lot less power. And second, they spool up at your ISP if they can't be delivered immediately (say if you only want to only turn your cellphone on for five minutes at the top of every hour to save even more power). If the power outage takes down your cell service ISP this won't work. But in that case, any VOIP provider you have would probably be down as well.