[Catspaw]

My very old fax machine didn't have a return number. If whomever sent this had one of those old machines, it'd be untraceable--unless the receiving fax machine had caller ID.

[Talkwire]

The phone company has a record of every call to and from every phone. These are the LUDs you hear Briscoe on Law and Order talk about. Local Usage Detail (LUD): Provides itemization of all calls terminating within Full Local Calling Area.