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 privacy was very controllable by me and nobody would think otherwise.
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 ;)
I know for me I stopped answering the phone altogether about a year or two before the do not call list went into effect. About literally 1/10th of the calls I receive to this day are legit calls I want to take.
I don't get how modern phones not having a busy signal forces you to let people contact you. If I really want to disconnect, I just turn stuff off. The only remaining difference is voicemail, but that's easily ignored. As for the concern, that's not the tool's fault, either, just how we use it. People get worried if you don't call back in a day because they've become accustomed to you doing so. I've got friends who I will miss a call from and not call back for a few days, and it's just fine. Not the phone's fault at all. The tool is still controllable by you - so long as there's a power switch, its in your control.
The telephone was a shared device, and as a child it was not mine. In college, it was shared with 4 other people (and still not mine). Then once married, it was shared with my spouse.
But now there is no more 'shared voice device.' It is not pratical to not own a phone. But all cell phones are not sharable. Hence, by default, either I carry a phone and the contact person knows explicitly I am ignoring them or I answer it. But when the only game in town was a simple telephone, a busy signal or no answer just meant nobody is available. Nothing else. But now, with my phone, I can no longer say I am not available, imo.
The introduction of voice mail at work is what killed the efficiency of telephone communication. It became very easy to let a call go to voice mail, and then claim you never got the message, or your voice mail wasn't working, etc. It was much harder when when a secretary took your calls and kept a log of messages.
Voice mail and then email (esp. auto-responding "I'm out of the office") became great work-avoidance enablers, whereas before the telephone was a great productivity enhancer.
So the "tool" to maintain privacy was very controllable by me and nobody would think otherwise.
Any Norton product dBase III