An operation is pending.

First off, make sure you have no other software or programs using the modem, or of you had say a fax program that was using the modem right before you got this error, that could be the cause.
1. close the other applications and try again.
2. reboot the computer and try again, ensuring the other appliations are closed.

If you are still having this error, there is a good chance some piece of software is trying to use the modem, try to determine if you have installed anything new lately that might be causing the issue, or possibly a virus, etc...