I'm a total noob and I'm trying to help converting a landline telephone style door intercom into a push to speak speaker intercom. So turning this into this. My country uses old-school systems and changing it for a new one that's just a speaker isn't possible in an old building. I'm trying to make a simple way to answer the door without picking up the "phone" and put it to your ear. My idea is to press a button to answer, another to open.

Anyway, I opened it up and it seems easy enough, I though. So, what I've tried and my problems:

• I tried soldering a new speaker to it to confirm my testing (its basically 2 cables: red/black, positive/negative) and it sounds in a minimum volume, just like the earphone.

• I tried adding in between an LM386 prebuilt module with a 9v battery and it doesn't seem to work, no sound comes out?¿?¿? It seems to get some electric interference so maybe its not as easy as I thought and the "red cable" is not just for audio?

• I tried just using that same lm386 with a normal audio input (from a jack) and the original speaker and it sounds amplified, just like I'd want to.

So now I'm a bit out of ideas. The lm386 seems to work but just not on the door phone output. But if I connect that same output to a speaker (tried a couple I had around) it does sound (really low volume). Maybe I need a simpler way to amplify that sound?

Thanks for your input, and again, I don't have much knowledge of electronics so please be patient.

EDIT: Picture of the explanation https://imgur.com/a8SAOYL