IP phone means you need to backup you modem too doesnt it??
Referring back to Doggy's question. Does your Internet modem have it's own battery back-up? And what about the neighborhood internet distribution point?. If they go down, then you will be without phone service even if your phone is still alive. Probably no way to know about anything beyond your walls, but if you need backup for both the modem and the phone then the easiest would be an AC UPS that both of those devices plug into.