I integrated a voltage inducer into a modem a while back, it's not difficult if you have some basic soldering experience, as well as a soldering iron and multi-meter on hand - you also probably need a razer blade (depending on your modem's internals).
First you'll need one of thesehttps://www.amazon.com/gp/aw/d/B011EBSK ... Mwebp_QL65
This boost board includes a POT so you can adjust it for either 9v or 18v (for PAL modems).
Other Parts you'll need:
~330 ohm resistor
0.47-1.0 uf capacitor
Diode (required because of the boost converter)
First things first you'll need to build the inducer, I recommend putting a small PCB (or a bread board) under the boost circuit (insulated with electrical tape) and a bit past the end of it towards the output side - this will allow you to have somewhere to solder your inducer circuit (you don't need to use a small PCB for this - but it looks a lot cleaner and should be much easier to work with that way).
The circuit you need to make is:
out(-) -> resistor -> capacitor -> diode -> out(+)
Make sure that the diode and capacitor polarities are right!
Once the circuit is put together you'll need to locate +5v and Gnd points on the modem PCB (near the USB jack) and solder those to the voltage inputs on the boost board to power it.
Now for the main event - the idea here is to connect one of the phone wires (the red wire usually) with the capacitor, in-line.
So first we need to find the red wire on the modem PCB, to do that you can either look at a phone cable to determine which pin on the RJ-11 jack the red wire contacts or strip a phone cable and use the exposed wire - we'll need to test for electrical continuity between the solder joints of the RJ-11 jack on the bottom of the modem PCB and either the pin in the RJ-11 jack or the stripped wire from a phone cable. Once we've done that we know now which solder joint in the bottom of the PCB connects to the phone line wire we want to induce the voltage on. This point will be the output of the voltage inducer circuit.
Next we hunt around the modem PCB (surely very near the RJ-11 jack) with our multi-meter - looking for a point on the board which is has electrical continuity with the solder joint we just located - once we find that we look for the board trace on the PCB which carries the signal to the RJ-11 jack. This is the modem's output signal - exactly what it puts on the phone line. The point we just found will become the input for the voltage inducer - but it won't do anything unless we first destroy the board trace that currently carries the signal. This is where a razer blade comes in handy - once you've located the signal trace and cut it (as deeply as you can) you should no longer be able to detect electrical continuity between the PCB underside joint and the modem's signal output - this is good! The modem can now be modded.
Finally, solder a wire to connect the 'input' point from above to one side of the capacitor in the voltage inducer and solder another wire from the other side of the capacitor to the 'output' point. This will restore the modem's connection to the RJ-11 jack - but this time with integrated voltage!
Finally - don't forget to measure the output voltage of the boost converter circuit and adjust it (using the POT) to about +9v (I actually run mine at about +10.5v which works great, but anything over ~+8.8v should be fine).
The above instructions should work for pretty much any modem - hope that's not too complicated of an explanation.