Error 680, there is no dial tone

But, there is... ;)

I have pulled out my hair on this one. Every time we try to dial out, we get this error message. Troubleshooting done thus far: (No DSL mind you, just regular analog phone line) Phone line works fine when plugged into a land-based phone, but swapped phone cords and tried multiple locations to rule out telco issue. Re-created the dialup connection; created a few new dial up connections; re-installed the modem; updated the driver for the modem; ran the modem helper & gets error 70: There is dial tone coming into the modem, but the modem cannot "hear it", tried system restore; and have even replaced the physical modem itself (and then repeated all the above troubleshooting steps again). I have spoken to 4 different Dell reps who, after an average of 3 hours, cannot seem to figure out what the issue is...(and insist that this laptop had a "PHONE" port AND a "LINE" port, when there is only one phone jack on it, and the dial up worked previously)

This is my frustration. To make it worse, we paid an extra $230 to have the extended "all-inclusive" warranty, including on-site service, but not one of the reps was willing to send someone out...

Any help or insight anyone can provide? Thank you profusely in advance!!