Configuring a modem in Windows XP for those like me used to use Debian can be a bit non trivial.
I got the Error 680: no dial tone. I know that it’s easy to solve this problem, just disable the “wait for dialtone” and put one comma before the number (wait for a second before dial). The non trivial part is to find where I can tell it. So, here it is:
Start->Settings->Control Panel->System and click on the Hardware tab and then on Device Manager.
Then select the modem from the list of devices. Right click and properties. Now on the Modem tab uncheck the Wait for dialtone