Emergency calls
Access Point Base Stations are also required, since carrying voice calls, to provide a 911 (or 999, 112, etc.) emergency service, as is the case for VoIP phone providers in some jurisdictions.[17] This service must meet the same requirements for availability as current wired telephone systems. Simply the phones must work if the AC mains grid is blacked out. There are several ways to achieve this, such as alternative power sources or fall-back to existing telephone infrastructure.