Impedance is AC resistance.
The mic is 4300 ohms and the resistor that powers it is 1k and is effectively in parallel so both together are 811 ohms.
The pot might be 19k and is in series with the 1k resistor to ground which is in parallel with the input impedance of the recorder that might be 10k ohms so their total is 20k ohms.
Then the mic circuit has a total resistance of 20,811 ohms.

A telephone mic probably rolls off frequencies below 200Hz.
The formula for the capacitor value is 1 divided by (2 x pi x f x R). Use a capacitor value from 0.047uF to 1uF.

 

Well, after a few days I finally figured it out.

The recorder uses the volume knob when RECORDING to change the RECORDING volume!

This is why I was getting no sound at some points and very loud sounds at others.

With a 17K pot I can trim the sensitivity pretty accurately and get a good audio quality!

Sounds like the " good ol days!".

Other than the voltage divider, all that was needed was a capacitor.

I noticed when I unplugged the mic when the circuit was live, I got a loud POP on the recorder. I hypothesized that it was the cap trying to keep the voltage up. I think I was right. Disconnecting the mic from the audio socket seemed to make a unnoticeable transition from recording to...well....not recording.

The capacitor was also changed to 10 micro farads and that AS WELL made an improvement.

I am indeed satisfied!

Thank you to all who contributed!