can a fax be sent when modem is in use?

Have an internal v.92 fax/modem and XP Home Edition.

I am dialed into my ISP and try to send a FAX.
The console shows the FAX status is "Pending".
The FAX is not sent until I disconnect the
modem from the ISP.

Must the modem be disconnected, and not in use,
in order for a FAX to be successfully sent?

Thanks,
NB