# Receiving fax billing events

T.38 fax full duplex channels consist of a single T.38 fax relay channel filter. T.38 fax relay filters can return billing events that show the status of T.38 fax transmissions. By default, T.38 fax relay filters do not return any unsolicited events. To enable this feature, the application uses mspSendCommand to send an MSP_CMD_FAXRELAY_CONFIG command to the appropriate T.38 fax relay filter. An msp_FILTER_FAXRELAY_CONFIG structure included with the command specifies an eventmask parameter. This parameter determines the types of unsolicited events the T.38 fax relay filter generates.

Applications can specify the following unsolicited event masks:

Mask | Events
FAXRELAY_EVENTMASK_REPT_FAX_BEGIN_END | Reports the beginning and ending of a fax session.
FAXRELAY_EVENTMASK_REPT_PASSED_PAGE | Reports successful passing of a fax page.
FAXRELAY_EVENTMASK_DEFAULT | Returns to the default filter configuration. No fax session events were returned.

Applications must invoke mspReleaseBuffer to release the buffers returned with T.38 fax unsolicited events.

For more information about structures returned with T.38 fax unsolicited events, refer to the mspunsol.h header file. For more information about sending commands to MSPP endpoint and channel filters, refer to the Dialogic® NaturalAccess™ Media Stream Protocol Processing API Developer's Manual.