You might try a software approach by going to *CONNECTIONS* and selecting *ALL*. This will show you all the connections installed on the PC. Highlight the *Dial Up* option and right click then select Delete. If their are more than one Dial Up connections delete those the same way. Now select *Create a New Connection* and follow the instructions.
Have you tested the Modem/cable? If that's faulty then it's a hardware problem and the equipment will need replacing.