It's not a miracle or a hack. Modems were sound bits of engineering and were based on international standards that allowed them to interoperate with each other

It's not a miracle or a hack. Modems were sound bits of engineering and were based on international standards that allowed them to interoperate with each other properly. They had clearly defined electrical and protocol standards. They may use the principle of a modem but there is no established standard for communication. I assume that you know about impedance and loading based on the above? If the internal resistance of the current loop suddenly flies up or down, is the circuit on the other end going to perform to the same? Nope. There will be less current available for the device. At that point it will work intermittently or stop working all together. Your microphone or earphones will still work quite happily though.