0 - DO_NOT_USE_UNKNOWN - When this attribute is set to 0, MTAS will not include Calling-Party-Address AVP when the P-Asserted-Identity is empty in the initial INVITE. 1 - USE_UNKNOWN - When this attribute is set to 1, MTAS will use 'unknown' as the Calling-Party-Address AVP when the P-Asserted-Identity is empty in the initial INVITE.