Hi, @Grayus I'm sorry about the delay getting back to you. The amplify button should boost the incoming speech volume on the handset when making or receiving a call and it will return to the default when the call finishes.
Have you tried increasing the default volume to level 5 to see if that makes it any better? I've included a link to the full user guide where it shows you how to adjust the volume settings.
Thanks M8 must have had a senior moment as I read the book and missed it, still a redial button would have made more sense and ease .