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.

I remember those. X sub C and C. I wish we did some experiments in school

 

Hello, Katelyn. Welcome to AAC.

This should really be a new thread, and I expect the moderators will make it one.

You could use a device like this: https://www.amazon.com/Telephone-Recorder-Memory-Landline-Office/dp/B00VE0X6PQ if you provide "talk power" to the phone which is about 9VDC when the phone is off hook.

But I would be inclined to use one of these: https://www.amazon.com/Viking-DLE-200B-Two-Way-Line-Simulator/dp/B004PXK314 and a good old answering machine. It would get a dial tone, then ring and connect, then announce and record.

It's not a terribly complicated setup but you might have to work at it a bit since you don't already no about phones and phone lines. Still, I think you could make it work.

Good luck.