Missed calls

Not receiving calls! I can't get hold of my wife through a normal voice call. Not just my phone calls but a few others who may call her. She has changed her sim card (which was suggested) but to date it is still giving het missed calls. When the person phone there is no signItal or response on her cellphone! She stays on a smallholding and there is a lot of dangers lurking about. It is frustrating if we can't get hold of her! What can we do?