This object specifies the forward signal sent to indicate that a backward signal just received could not be processed or was not recognized/defined by this end. This is effectively an R2-level error/NAK response, so a rising count of this signal on a trunk points to a protocol or country-variant mismatch between the two connected exchanges; they are sending each other signals the other side does not understand. If a terminating exchange configured for one R2 country variant receives a backward signal request that its variant does not define, it would respond with this forward signal rather than the expected data, flagging the incompatibility.