(1) Time.

(a) Within 20 days of the date of service of the complaint by the commission, the defendant or defendants shall answer the complaint. The commission may require the filing of an answer within a different time.

(b) Requests for an enlargement of time to answer a complaint shall be directed to the commission, in writing, and a copy provided to the complainant. The request shall indicate complainant's acquiescence to said extension of time or the measures taken by the defendant in his unsuccessful effort to obtain such acquiescence. The commission shall notify the parties of its ruling.

(c) If an amendment to a complaint is filed before receipt of the answer, the defendant's time to answer the complaint shall be 20 days from the date of service of the amendment, unless otherwise directed by the commission. Amendments to a complaint made subsequent to the filing of an answer need not be answered by the defendant.

(2) Contents. The answer must admit or deny each material allegation of the complaint or allege insufficient information on which to admit or deny the same. It shall set forth any new matter relied upon as a defense and shall be so drawn as to fully advise the complainant and the commission of the particular grounds of defense. The filing of an answer will not be deemed an admission of the sufficiency of the complaint and shall be without prejudice to the rights of a defendant to thereafter file a motion to dismiss the complaint for failure to state a cause of action. In rate complaints against a utility, all averments will be deemed denied and the matter will be of issue if the utility files no answer.