Even if I replace the speaker from a telephone, and the keypad stops working a month later, I have voided the warranty already by doing the speaker change as they can’t know if the now not working keypad was done because of you or a failing unit.

This is false. They have to PROVE that the repair that you made caused the keypad to fail in other to void your warranty, at least in the USA. Most people are misinformed as you are however, and they’ll TELL you that’s the case to make you go away, despite it being illegal.

en.wikipedia.org/wiki/Magnuson-Moss_Warranty_Act that