Replace microphone signal with an audio signal taken from my phone
I wouldn't have especially high hopes of success. Microphones can be so tightly built into the system that it's even mechanically impossible to make own modifications. The needed wires can be buried under very difficult to disassemble parts on the circuit board and the sheer density of wires can be to high. Think how do you tap to one wire which is one in a bunch which has 4 wires in a millillimeter wide zone. I do not claim you'll meet this but prepare for it. 

If the mechanical problems are solvable we do still know nothing about the needed signals. Even in case they are old fashioned electret mics they can get some supply from the rest of the electronics and disturbing it can cause a now unpredictable error condition. At least headphone signal can be as much as 1000x too high, it needs an attenuator.

If you are going to continue get local help. An experienced electronics hobbyist with your Alexa can well be interested in this journey if he is free from all responsibility of caused consequences.