this is the way
I don't use netzero anymore (since I have DSL 

 ), but this worked with couple of friends...
Set up a completely new netzero account. when it asks you for the phone no., enable the 
To disable call waiting box and 
change the "*70" to "*67". that will do it. just make sure to create a new account once in a while.
Why does this work? NetZero's Cisco routers 
comes with caller-id support. Simply by dialing *67 to disable caller id for the person you calling TO, the routers can't tell your phone number and thus can't track your individual phone line/computer.