Extracting speaker, microphone and hook signals from a two-line telephone
It looks like you will need to hold the speaker on your ear because its power amplifier is missing. Then the hybrid matching to the phone line will not be perfect so you will have a noise into the mic will come out the speaker and the mic will pick it up and the sound will go around and around as acoustical feedback howling.
Very old speakerphones used a push-to-talk pushbutton that disconnected the speaker when you talk and disconnected the mic when you listen.
The old and modern mics need to be DC powered.