The battery-reversal command applies to FXO and FXS voice ports. On Cisco 2600 and 3600 series routers, only analog voice ports in VIC-2FXO-M1 and VIC-2FXO-M2 voice interface cards are able to detect battery reversal; analog voice ports in VIC-2FXO and VIC-2FXO-EU voice interface cards do not detect battery reversal. On digital voice ports, battery reversal is supported only on E1 Mercury Exchange Limited Channel Associated Signaling (MEL CAS); it is not supported in T1 channel associated signaling (CAS) or E1 CAS.
FXS ports normally reverse battery upon call connection. If an FXS port is connected to an FXO port that does not support battery reversal detection, you can use the nobattery-reversal command on the FXS port to prevent unexpected behavior.
FXO ports in loopstart mode normally disconnect calls when they detect a second battery reversal (back to normal). You can use the nobattery-reversal command on FXO ports to disable this action.
The battery-reversal command restores voice ports to their default battery-reversal operation.
If an FXO voice port is connected to the PSTN and supports battery reversal, use the battery-reversalcommand with the answer keyword to configure answer supervision. This configures the FXO voice port to detect when a call is answered in order to provide correct billing information.
If the voice port, PSTN, or PBX does not support battery reversal, do not use the battery-reversalcommand because it prevents outgoing calls from being connected. Use the supervisoryanswerdualtone command instead.
If an FXO port or its peer FXS port does not support battery reversal, avoid configuring battery-reversal or battery-reversalanswer on the FXO port. On FXO ports that do not support battery reversal, the battery-reversal command can cause unpredictable behavior, and the battery-reversalanswer command prevents calls from being answered. To ensure that battery reversal answer is disabled on FXO ports that do not support battery reversal, use the nobattery-reversal command.
The following example disables battery reversal on voice port 1/0/0 on a router:
The following example enables battery reversal to provide answer supervision on voice port 1/0/0 on a router: