App cannot access modem

I have a client who uses a government supplied app to
communicate with them via modem. It is an old style app
that needs to access the modem directly rather than using
the Windows dial-up networking. It is installed on a
Win2K Pro machine.

For some reason, even though it worked a few weeks ago
and the client claims they didn't change anything, it can
no longer access the modem. It says the port is not
available. The modem works fine through a dial-up
networking connection or with hyperterminal. I confirmed
that the modem is on COM1 and the app is trying to use
COM1. Apparently, something in Windows is tying up the
port. I would appreciate any help anyone can provide.

Thanks in advance,
Andy