Our alarm system couldn’t have been simpler.  We still had the old crank telephones.  While the system was poor, with no expectation of privacy, it was perfect or us.  I remember that there were 21 people on my line, and that you heard every ring.  It was early call forwarding, for if you were at someone else’s home that was on your line and you heard your ring you could answer.  We checked with the telephone operator to confirm that five long rings was not used, and adopted that as the fire alarm.  This stayed in effect for a number of years until the phone company replaced the old crank system and made us move on to the next phase, which was using a one way call in only telephone placed in selected homes.