Stepping up from 12vdc to 100vdc/-100vdc
The "instructable" in update section really generates the needed voltage, but it's not practical. It's a teacher's demo for the basic DC voltage boosting with an inductor and a switch. Every operating cycle pushes a portion of new charge to the output capacitor and the output voltage grows. The growth stops when one of the following happens:


the load takes as much current as the inductor pushes as an average
the diode starts to conduct reversely due its limited voltage range
the output capacitor or the load break down


What makes it impractical:


there's no regulation which limit the output voltage to a safe value by stopping the switching as long as it's not needed
there's no security measures which prevent the switch current to grow dangerously in case the switch is ON too long time. In inductor the current grows gradually from zero to the limit caused by Ohm's law at rate U/L.
you need 2 circuits; one for plus and one for minus.


Use at least properly controlled switching regulators. They have been around so long time that they can be considered to be mature. 

The next is effectively a suggestion to redesign the ringer circuit. 

One DC voltage is enough if you make pulses of it and remove the DC component with a capacitor. Landline phones can have that capacitor, so switching the voltage between 0  and 100V where 0 means active pull down can be an acceptable ring voltage. But that's not quaranteed. Old phone specs had AC ring voltage added to the DC.