Read the forum guidelines
'Why wasn't i offered Battery backup for 911 service: I would like to purchase one to have "FCC Required" 911 Service during Power outages?'
Federal Communications Commission.
In this document the Federal Communications Commission (FCC or Commission) adopts rules to promote continued access to 911 during commercial power outages by requiring providers of facilities-based, fixed residential voice services, which are not line powered, to offer subscribers the option to purchase a backup solution capable of 8 hours of standby power, and within three years, an additional solution capable of 24 hours of standby power. The item also promotes consumer education and choice by requiring providers of covered services to disclose to subscribers the following information: availability of backup power sources; service limitations with and without backup power during a power outage; purchase and replacement options; expected backup power duration;) proper usage and storage conditions for the backup power source; subscriber backup power self-testing and monitoring instructions; and backup power warranty details, if any.
Pretty sure Cox sells/rents the battery now. See link below for more info.