It gets quieter because of the transmission loss plan. Historical reasons, basically. Since analog connections lose power with distance, there was defined an

It gets quieter because of the transmission loss plan. Historical reasons, basically. Since analog connections lose power with distance, there was defined an acceptable maximum loss for each segment in the network. If a line came in too hot, a "pad" was added to dissipate some power at the receiving end. If it was too low, the circuit was out of spec and had to be re-engineered. At the other end of the telephone switch, an amplifier was added to jack up all the outgoing signals to the proper amplitude for the transmission network. All this amplification and loss was necessary to make sure that all telephone calls had about the same volume. The network didn't all switch over to digital switching and transmission in one day (in fact there are some analog interoffice circuits still in use to this day in Alaska), so we retained some vestiges of the loss plan. It's pretty likely that your PBX system connected to the network over analog lines anyway. :)