There are a number of reasons why they may write to you other than as a warning, they may ask you to attend the hearing (without the additional warning) if they think it is to your advantage to do so, they may also write to ask permission for a copy of your medical records.
But usually they will not contact you at all and the next communication you receive will be the Decision Letter.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems