In the real old days, my modem would never connect at better than 24K bits (when theoretically you could get 56K).
Complained, had them check it, couldn't do anything. In fact they told me all they would guarantee was 2400 buad.
when i contracted for DSL, right before they installed they upgraded my line to the house and I got 56K for a day, all day.
Maybe your modem got wind of the upgrade and in a vain attempt to avoid retirement and possible landfill internment it decided to play nice; finally realizing that continually frustrating the owner, while entertaining at times, leads to eventual self destruction.
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