VMs and ISDN modems

Hi. I have a VM but I would like to be able to use an ISDN modem with.

We are using ESX 2.5.3 on this server at the moment.

How do I get the VM to 'see' the ISDN modem, I am hoping this is possible, just trying to find some documentation.

Note: The VM would stay on one ESX server and would not be migrated (if DR situation, it has been agreed they will not be able to access this modem)

Thanks

Debbie