The tapi/line device class consists of all line devices. You access these devices using the TAPI line functions.
The lineGetID function fills a VARSTRING structure, setting the dwStringFormat member to the STRINGFORMAT_BINARY value and appending this additional member:
DWORD dwDeviceI; // line device identifier
The dwDeviceId member is the identifier of the line device associated with the line handle given by lineGetID.
The phoneGetID function also fills a VARSTRING structure, setting dwStringFormat to STRINGFORMAT_BINARY and appending this additional member:
DWORD adwDeviceIds[]; // array of line device identifiers
The adwDeviceIds member is an array containing the device identifiers of all line devices that are associated with the phone device. If there are no associated line devices, phoneGetID returns the PHONEERR_INVALDEVICECLASS value.