How to make a voice assistant for talking on the phone?

Question

Speech recognition

how to make a voice assistant for calls.
Let me explain, there is an idea, when you press the button, there will be a call to a certain number, after the person picks up the phone, the assistant will begin to speak the prepared text.
There is an approximate understanding, that is, to make a simple assistant who will understand the words of his call, like: yes, hello, and so on, after which the text will speak.
But, how to fasten it to the dialer?
I hope I explained well.