SG electret microphone static impedance
Most of the ECMs I've used/seen operate between 1V - 10V.

Also, the frequency response of these guys vary quite a bit. Some ECMs and/or MEMS mics are optimized for a particular bandwidth. For many telephony applications (300Hz - ~3kHz) you'll find a high frequency rolloff, which may be what you're seeing. May want to try out a couple other mics.