AI -
Yes, ringer volume controls were common on electronic and digital push-button phones by 1985. Whether it was a physical switch (Low/High/Off) or a rotary dial on the base, manufacturers—such as British Telecom (BT) in the UK and Western Electric in the US—routinely included these to let users manage their electronic ringers.
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This is also interesting. People have said Jeremy could not have answered his own call as the next door neighbours would have heard a ringing phone.
I have posted photos of phones then. They either had the High/Low/Off option or a button you could push up and down.