MacMan, i have a simmilar problem with the machine not properly disconnecting from the net if the call drops out. I dont know about noise on the line, as it's purely a data line. Have you configured your mac's ethernet port (if you have one) and if so, did you have any issues with it?
PowerMac G5 Dual 2.0Ghz | 1Gb | 250Gb | Bluetooth | NVidia GeForce 6800 Ultra DDL 256Mb | 20" Cinema Display | MX1000 Wireless Laser Mouse | OS X 10.3.9
PowerMac G4 400Mhz | 832Mb | 40Gb + 120Gb | OS X Server 10.3.8 - Web Dev, Proxy, Mail, NAT, Firewall, Backup
Netgear Gigabit Switch | Sony Ericsson P910i Smartphone | iPod Colour 60Gb