What if supervisory tones do not work correctly?

If callers do not hear the supervisory tones, like busy or number unobtainable, check the optional fourth parameter to the protocol variant.

The handling of supervisory tones varies from place to place. In most cases tones like number unobtainable, busy and disconnected are generated at the caller's exchange. The ringback tone is generated at the called party's local exchange. This is not always the case, so the software provides options for how supervisory tones are handled.