The Telephone with Dial Tone and associated Busy Signal. A busy signal meant nobody could contact me; Also, by either not answering the phone or by simply taking the receiver off the hook I controlled interruptions. And, nobody became concerned if they did not hear from me in an hour, 1/2 day, day or even week. And I had no concern if I did not hear from others either in that timeframe. So the "tool" to maintain priv…
Maybe it's just me but I feel that it was also much easier to get to actually speak to someone on the phone back then. We usually knew when we could reach someone and call during those timeframes. If we didn't get a reply we knew they were out and call the next day. Nowadays it's much more rare to get an answer when calling a mobile phone. Either the person has left it in the next room or it's out of battery, it's in silent mode and the person doesn't hear it, etc. So people usually tend to resort to text messages, which is a highly ineffective way to communicate in many situations.
Also old telephones never took the initiative to call someone from their owner's pocket ;)