The PHONEEXTENSIONID structure describes an extension identifier. Extension identifiers are used to identify service-provider specific extensions for phone device classes.
typedef struct phoneextensionid_tag {
    DWORD  dwExtensionID0;
   DWORD  dwExtensionID1;
    DWORD  dwExtensionID2;
    DWORD  dwExtensionID3;
} PHONEEXTENSIONID, FAR *LPPHONEEXTENSIONID;
 Not extensible.
Extension identifiers are generated using an SDK-provided generation utility.
TSPI_phoneNegotiateTSPIVersion