Under normal conditions, an ISP can 'see' the modem from their offices and can track down the problem. Regardless, and I manage almost 50 different remote locations, 20% of them DSL, they usually want to send a tech out to the location to verify that the modem is inoperable.. Occasionally, I can just get them to send me out a new modem if I think that is the problem... Chances are you are right and the problem is at the CO, but they send them out anyway.. I know that ATT here in the midwest knows how to WASTE valuable resources and money..
I would not waste your time arguing with them... :)
FE