Disable volume buttons for PHONE CALLS

My 96 year old mother has an iPhone and turns the volume down by the way she holds it. She’s 96 and in assisted living and loves FaceTime (her whole family uses apple phones). But she consistently turns down the volume without knowing it. Then she can’t hear us no matter how loud we scream. Can we disable the volume button for this situation? Our family is at wits end because she hangs up and calls back many, many times because she thinks the phone isn’t working. Why can’t apple offer a senior citizen phone???