I believe this is what hathway also does. What i dont understand is, why cant they use the caller id feature to detect the phone number which has the DSL and authenticate based on that.
CallerID like the mac address can also be spoofed. All CallerID really is, is a bunch of key-tones, all transmitted just before sending the ring along. The mac address on the other hand is more visible on the network software side.
Cheers,
Amol Hatwar