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You might have been up against a ground start phone line.[1] Apparently, some designs gave access to the powered wired from the phone line, and grounding this would trigger the dial tone from upstream.[2]

[1] https://en.wikipedia.org/wiki/Ground_start

[2] https://groups.google.com/g/comp.dcom.telecom/c/weJ0FW0Njb4

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Not sure about any of that - there was always a dial tone. But, what they had done with these phones was not let any sound from the mouthpiece go into the phone until all the appropriate coins were inserted. You could tell right away if it was a phone that this would work on or not by blowing into the mouthpiece. If you heard the wind sound in the earpiece, it would work, if no sound was picked up by the mouthpiece, it would not work.

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