Usually this error occurs when there is any application that is trying to make a connection or when the user is already logged in and tries to reconnect.
1- Restart the computer and reconnect
2- Win 9x / ME: Reinstall Winpoet and Communications
Win NT / 2000 / XP: Sign in services, within administrative tools and restart it, besides disabling Red plate connected to the modem and re-enable it.
3- Win NT / 200: Reinstall Winpoet
Win XP: Delete the connection and recreate it.