Remember that calling them back or pressing something "to be added to our do not call list" only serves one purpose -- to tell them that their random-dialed call has found a real, live prospective sucker, so they can mark your number as such.
Most often these calls don't come from actual phone lines... they "borrow" a legitimate number to use as a Caller ID number, but are actually originated from computers which hack into the phone system, in ways similar to the way Skype and Vonage link from a computer into the phone system.