Analog Telephone Frequency Response Curve
Carbon mic's were pressure controlled resistive devices that produce variable current when driven by a voltage source. They had poor frequency response but the side tone could be cancelled out or adjusted to a minimum easily, so your own voice was minimized on your earpiece. (sidetone) But the freq response was as it is defined 400~3.2Khz today.  with 8kbps digital rates present day telephony is limited by a 7th order "brick filter" at 4KHz to satisfy requirement to block all image signals> 4kHz. 

I do not have curves for skirt steepness on carbon mics.