If you can hear a lot of static while you are on a call, more than likely this can be fixed by replacing your curly cord that connects the phone on the desk to your handset.
If you replace the cord and find that that the problem persists or if the static happens on a specific line (e.g., all calls on line 2) you should contact your service provider to report the issue so they can check to see if there is any trouble on their end.