TSPI_lineMonitorTones function (tspi.h)

The TSPI_lineMonitorTones function enables and disables the detection of inband tones on the call. Each time a specified tone is detected, a message is sent to the client application through TAPI.


LONG TSPIAPI TSPI_lineMonitorTones(
  HDRVCALL                hdCall,
  DWORD                   dwToneListID,
  DWORD                   dwNumEntries



The handle to the call for which tone detection is to be done. The call state of hdCall can be any state except idle.


The unique identifier for this tone list. Several tone lists can be outstanding at one time. The service provider must replace any old list having the same dwToneListID with the new tone list. If lpToneList is NULL, the tone list with dwToneListID is simply dropped. In any case, other tone lists with different dwToneListIDs are kept unchanged.


A list of tones to be monitored, of type LINEMONITORTONE. Each tone in this list has an application-defined tag field that is used to identify individual tones in the list for the purpose of reporting a tone detection. Tone monitoring in progress is canceled or changed by calling this operation with either NULL for lpToneList or with another tone list. The service provider must copy the tone list into its own memory for later reference, rather than simply retaining the pointer into application memory.


The number of entries in lpToneList. The dwNumEntries parameter is ignored if lpToneList is NULL. TAPI does not validate this parameter when this function is called.

Return value

Returns zero if the function succeeds, or an error number if an error occurs. Possible return values are as follows:



This function returns zero (success) when tone monitoring is correctly initiated, not when tone monitoring is terminated. As with media monitoring, tone monitoring remains in effect for a given tone list until that tone list is explicitly disabled by calling TSPI_lineMonitorTones with the same dwToneListID and another tone list (or a NULL tone list), or until the call transitions to idle.

Although this function can be invoked in any call state except idle, tones can typically only be detected while the call is in the connected state. Tone detection usually requires computational resources. Depending on the service provider and other activities that compete for such resources, the number of tones that can be detected can vary over time. Also, an equivalent amount of resources can be consumed for monitoring a single triple frequency tone versus three single frequency tones. If resources are overcommitted, the service provider returns LINEERR_RESOURCEUNAVAIL.

The service provider monitors for all tones in all tone lists concurrently. When a tone is detected, each matching tone from each tone list is reported separately using a LINE_MONITORTONE message. Each tone report includes both the tone list identifier and the application-specific tag. Some service providers may not be able to discriminate very close tones, so that multiple matches may be reported even for tones whose descriptions are not strictly identical.

Note  TSPI_lineMonitorTones is also used to detect silence. Silence is specified as a tone with all zero frequencies.
The corresponding function at the TAPI level does not include a dwToneListID parameter. The inclusion of this parameter at the TSPI interface allows TAPI to forward the union of all tone monitoring lists from all applications to the service provider, while still retaining the ability to filter and forward the tone detection events according to application. This gives service-provider designers the maximum flexibility to determine the degree to which they can discriminate very close tones, because TAPI makes no assumptions about what tone descriptions are considered identical.


Target Platform Windows
Header tspi.h

See also