Why telephone dc voltage drops to about 8 volts?
The telephone handset impedance is about \$600\:\Omega\$. There is also any output impedance at the central office (not uncommonly \$1\:\textrm{k}\Omega\$), plus a LOT of wiring going from the central office to the remote location. The wire is often 26 gauge and can amount to \$1500\:\Omega\$ or more.

Battery voltages at the central office can be higher than \$48\:\textrm{V}\$, too. In my own location, the battery voltage was almost \$90\:\textrm{V}\$. I believe the specification I read noted a figure of around \$105\:\textrm{V}\$ as one of the higher likely battery voltages.

Off-hook, the phones at various locations here varied from about \$6\:\textrm{V}\$ to about \$12\:\textrm{V}\$ (measured by me.)

I have the most recent official specifications document for the US phone systems (the specification includes lots of operation regions, since they had to accommodate various central office equipment manufacturers) somewhere here in the house and could, if asked, find and refer to specific charts and text, if seriously needed.



I do also recall that the specification specifically suggested an on-hook handset impedance of \$20\:\textrm{M}\Omega\$ and that in no case should the entire location's on-hook impedance go below \$5\:\textrm{M}\Omega\$. This was why I secured the specification, in fact. I was designing a circuit to show if the circuit was available, powering the entire circuit and LED from the nearly inconsequential current available. (That was a problem, but an achievable one. I had to use a relaxation oscillator that just barely trickled charge to a capacitor and then used a self-triggering SCR and some added components to shape and distribute the accumulated energy for the LED pulse.)