(a) Unless otherwise provided by law, all parties shall be given written notice of hearing by registered or certified mail with return receipt requested at least fifteen days before the hearing.
(b) Unless otherwise provided by law, if service by registered or certified mail is not made because of the refusal to accept service or the board or its agents have been unable to ascertain the address of the party after reasonable and diligent inquiry, the notice of hearing may be given to the party by publication at least once in each of two successive weeks in a newspaper of general circulation. The last published notice shall appear at least fifteen days prior to the date of the hearing.