How would you increase voltage to drive an LED yet also keep it limited?
Look at  Respawned Fluff's comment.
He's right on. You could use an Op Amp as a comparator and when the voltage level is exceeded, the output of the Op Amp can drive the LED. You could then control the voltage level with a pot. You should, however probably include a small buck converter to step the voltage down form 48v to like 5v or 3.3v to run your circuit off of. With that said, there is probably a power rail somewhere on the phone's pcb that has somewhere around 5v or 3.3v on it to run things like the microcontroller. You could probably tap off of that since you'll only be drawing a few mA. Finally, if you do you an op amp, it's inputs are usually high impedance meaning you will draw very, very little current from that speaker output.

Maybe something like this: (Please correct me if I'm wrong here)







simulate this circuit &ndash; Schematic created using CircuitLab

PS. That -530 ohm resistor (R4) was supposed to say 470 to 1K ohm. 
And the actual value of R3 will depend on your V+ and your max voltage going to the speaker.