TAPI - PASSTHROUGH and MODEMSETTINGS

lineInitialize()
lineOpen(m_hLineApp, dwDeviceID, &hLine, dwAPIVersion, 0, 0,
LINECALLPRIVILEGE_OWNER, LINEMEDIAMODE_DATAMODEM, 0);

Fill out a LINECALLPARAMS struct. Here's an example:
Note the use of dwBearerMode = LINEBEARERMODE_PASSTHROUGH. That's the key to making it work.

LINECALLPARAMS CallParams;
ZeroMemory(&CallParams, sizeof(LINECALLPARAMS));
CallParams.dwTotalSize =  sizeof(LINECALLPARAMS);

// This is where we configure the device to allow us direct access to the modem in command mode

CallParams.dwBearerMode = LINEBEARERMODE_PASSTHROUGH;

// This specifies that we want to use only IDLE calls and
// don't want to cut into a call that might not be IDLE (ie, in use).

CallParams.dwCallParamFlags = LINECALLPARAMFLAGS_IDLE;

// We will assume there is only one address and use it (normal modem)

CallParams.dwAddressMode = LINEADDRESSMODE_ADDRESSID;
CallParams.dwAddressID = 0;

// Unimodem ignores these values.

(CallParams.DialParams).dwDialSpeed = 0;
(CallParams.DialParams).dwDigitDuration = 0;
(CallParams.DialParams).dwDialPause = 0;
(CallParams.DialParams).dwWaitForDialtone = 0;

// We're using lineMakeCall to open a "call" but it doesn't actually
// dial when dwBearerMode = LINEBEARERMODE_PASSTHROUGH. The
// "passthrough" call is used to set modem parameters such as baud rate,
// parity, data bits, etc.

lineMakeCall(m_hLine,      //  Handle to the open line device on which a call is to be originated.
             &m_hCall,     //  Long pointer to an HCALL handle.
             "",           //  the telephone number
             1,            //  Country Code
             &CallParams); //  the structure you just set up

//Wait for the LINE_REPLY...

lineGetID(hLine, 0L, NULL, LINECALLSELECT_LINE, lpVarString,"comm/datamodem");

hCommFile = *((LPHANDLE)((LPBYTE)lpVarString + lpVarString -> dwStringOffset));

GetCommConfig(hCommFile, commconfig, &size);

modemsettings = (MODEMSETTINGS*) &commconfig->wcProviderData;

//Do whatever you need to do...

SetCommConfig(hCommFile, commconfig, size);

lineGetDevConfig(0,lpVarString,"comm/datamodem")

lineDrop()

lineSetDevConfig(0,lpVarString",comm/datamodem")

//Do your normal call using lineMakeCall