The Vonage Quick Start Manual states “use of a surge protector is recommended”, but does not indicate what will happen if the power goes out and you need to call 911. Not much.
This is a common VOIP issue. Imagine you are in a severe rain storm or ice storm and the power is knocked out. Unlike older landlines, your VOIP phone system will go down unless your router, modem, VOIP adapter, and phones stay powered.
If you have a charged cell phone with good coverage, you may not have to worry about this issue. A good alternative would be to have an extended run time uninterruptible power supply (UPS) like the Battery Backup Power, Inc. 600 VA Line Interactive unit. Based on the average wattage of modems, routers, and VOIP adapters, it will power all of these devices for approximately 45 minutes automatically when a power outage occurs. Just plug the UPS into a wall socket and your modem, router, and VOIP adapter into the UPS. It will do the rest.