Is My Microphone Capsule Dead?

Most phone mics have automatic gain control, active noise cancellation, and other processing which makes having a sensitivity spec somewhat useless.

Additionally you don't care about a mV/Pa spec since you do not have access to the analog signal anyway. You might want something like dbFS/Pa.

And to make matters worse, there is likely a wide variation from phone to phone. Even with phones of the same make and model. Phones are not known for having high quality microphones. It wouldn't surprise me to see a 6 dB variation between the same model of phone.

You'll have to do some experiments and measure the sensitivity. Even then, don't expect things to be good or consistent.

Your circuit might almost work if you replace the microphone with a old carbon powder type. This is what old telephones had in the mouthpiece. That type of microphone looks like a variable resistor to the circuit, not the tiny voltage source your microphone probably is. With a variable resistor, you can just put it in series with the speaker and a battery and probably hear sounds picked up by the microphone in the speaker. If you can find a really old telephone with mechanical ringer, it might have a carbon mic in it.

Otherwise, microphones you get today produce a tiny varying voltage. This must be amplified to drive a speaker. Note that some types of microphones, called electrets, additionally need a few volts applied to it with a few kΩ imedance to produce that small varying voltage. Electrets often come in small cartidges like what you show, so you may have such a mic.