Using ISDN to call a gsm-modem with C#?

Question

Hi,

I'm developing an application which communicates with external devices over the serial port and by extension calling a gsm-modem with analog modem or connected mobile phone.

It doesn't present a problem with all the AT commands and handling the com port, but now i am asked if its possible to implement calling the external gsm-modem with a ISDN card / modem.

Now is there a way to do this and especially to preserve or at least emulate the serial communication?

Thanks in advance for any help

Dave

Edit: I am using VS2005