Battery Reversal is similar to CPC, except with Battery Reversal, instead of an open loop for 500ms, the talk battery is reversed for 500ms. Battery Reversal is still used in some countries, but is no longer in use in the United States. U.S. equipment may or may not recognize battery reversal. FCC registered telephone equipment is required to operate correctly on either polarity.
Some FXS equipment (usually manufactured for the non-US market) only sends a battery reversal signal. If those FXS devices are connected to equipment that does not recognize the battery reversal, the device will not hang-up the line at the end of a call, until the call times-out or the device is reset.
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