BT phones have for ages used automatic gain control to compensate for the line loss. Short lines with low resistance pass more current, which was used to reduce the gain in the phone, and conversely long lines the higher resistance pass less current, and the gain in the phone would be much higher. This was done with funny transformer circuits in the phone where the DC line current was used to saturate the core (I'm going back to the days when BT owned your phone and you had to rent it, and it had a dial and bells in it;-).
Many other countries did not have this automatic gain control in the phones (France Telecom is certainly one example). This meant that if you just swapped phones between lines, you might end up with a phone which you couldn't hear, or one which blasted out your eardrum. When the phone is properly installed, the gain has to be set by the engineer manually to match the line loss.
I guess modern electronic phones are all automatic gain control now.