sometimes it happens when dialling the wrong telephone number. And check if the handshake is OK.
And also check your modem. If the modem is ok, try uninstalling and reinstalling it and reboot your PC. Make sure to install the correct modem.
Also check your connectoid properties and the network settings. and try increase the connection timeout periods.
Otherwise, try contact your ISP and ask for the DNS1&2 and use these to connect.