said by yyzlhr:If managed cable VoIP isn't working for you then an unmanaged VoIP solution probably isn't going to work if you're concerned about it lasting through an extended power outage.
My recommendation would be VoIP and a cell phone that runs on the Bellus network for emergencies
having your own VoIP ATA with a service like voip.ms combined with a smartphone that gets LTE where you can load any mobile app is probably a good system. simply use the mobile app has fallback if you've lost power for too long and your UPS goes down, voip.ms can be easily configured to ring your ATA device first then your mobile app.