As she doesn't live alone, and this is a problem only when her carer isn't there, all I can suggest is that the carer disables the phone every time he/she leaves. Of course, that would mean that she couldn't call anyone when the carer wasn't there. To be honest, the issue appears to be less about the phone, and more about her being left alone. It is possible she has reached the state where, due to her anxiety, she should NOT be left alone. Most people get to this point due to lack of mobility or safety issues, but anxiety of this magnitude shouldn't be discounted either.