In order to fix error 678, the cause of the error must first be identified. Possible causes include dialing the wrong number, the inability for the modem to get a dial tone and the modem at the other end not answering.
Other causes include a problem with the service provider or the modem driver being incompatible with the Windows version being used.
The modem log can be checked to ensure that the number being dialed is correct. The modem log can be found in the Hardware and Sound section of the Control Panel under Phone and Modem Options.
If the modem dialing is not heard, it is probably not getting a dial tone. Ensure that the phone line is functioning properly. Telephone service providers may be able to provide assistance.
To test whether the modem at the other end is answering, dial the number from a regular home phone. The modem is answering if modem sounds are heard. Contact someone at the destination computer if modem sounds are not heard.
Contact the manufacturer of the modem to ensure that the version of Windows being used is compatible with it. Internet service providers can also be contacted to inquire whether or not there are any current issues with service.