Since you are directing unwanted calls, perhaps any old answering machine could be attached. You could include no greeting or one that says something like "your message will be ignored", let them record a message of limited length and never empty the messages from the machine. After the memory of the unit is full of whatever, the machine might simply report "message space is full" or not allow any more or whatever. But in any case, do you really care how long it takes between pick-up and hang-up of these unwanted messages? If not, then any old answering machine should work.