RAS dial fails on modem

We have a USB device which presents as a bus with serial port and modem
child devices. Configuring a serial port as a modem allows us to
successfully dial out using RAS but using our modem devices to dial out

The modem child devices are the same code as our serial port devices except
that they are installed as lower filters to modem.sys. On installation two
symbolic links are created by windows, one to the PDO and one to the FDO.

The problem occurs during a RAS dial.

Initially unimodem opens our FDO, apparently using the symbolic link to our
PDO with \Tsp appended, the following debug is displayed:
16 - UNIMDMAT: Opening \.\Data Modem\Tsp
Where “Data Modem” is a symbolic link to our PDO.

After a successful CONNECT this happens again for \Client as follows:
16 - UNIMDMAT: Opening \.\Data Modem\Client
16 - UNIMDMAT: UNIMDMAT: Failed to open (\.\Data Modem\Client). 00000005
This time the driver is not entered and the connection is then terminated.

What is the explanation for this and what is causing the failure?

We are testing on WXP SP2 Checked but have also tried W2K.