Electrical power is required for non-line-powered voice services like Connected Voice to work. If there is a power outage, VoIP service will not process calls, including those to Emergency 911.
With active batteries, a battery will provide temporary backup power during an outage, allowing basic voice services to keep functioning, including the ability to dial 911. Please note that in the event of an outage that impacts the service network as well, the battery may not guarantee functionality.
You can purchase a battery backup unit from most major electronics suppliers.