Electrical power is required for non-line-powered voice services (including Digital Home Phone) to work. If there is a power outage, the services will not process calls, including those to Emergency 911.
With active batteries, the BBU 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 BBU may not guarantee functionality.
You may purchase necessary battery backup solutions from CenturyLink, or you may get a BBU from another supplier and install it yourself. When you talk to a CenturyLink service agent, you can ask what battery or batteries you may need for your type of service.