Tomfoolery is better than malice
Ah the happy days of fax shenanigans.
Drawing a sinusoidal cycle in pencil on a bit of paper, gumming the ends together to make an endless loop, and sending it to head office, who got a whole roll of paper with a sine wave on.
Then ringing up two hours later, in the certain knowledge that some lowly functionary would have binned it, and saying "did you get my banner for Sir Leo's presentation this afternoon?"
Or sounding a message on pirated Buckingham palace letterhead asking for someone to contact the Met about security arrangements for tomorrow's royal visit - giving the phone number of a Chinese restaurant.
Most fax machines printed a header with the phone number of the source - but that wasn't caller ID, it was supplied by the machine at the other end. I discovered that Panasonics would let you programme anything you liked, such as "Embassy of the GDR" or strings of oriental characters . So all sorts of unsettling suggestions were possible.