Any employing unit, not otherwise subject to the provisions of the North Dakota unemployment compensation law, which files with the bureau its written election to become an employer subject to such provisions for not less than two calendar years, upon the written approval of such election by the bureau, shall become an employer subject hereto to the same extent as all other employers as of the date stated in such approval. Such employing unit shall cease to be subject to the provisions of the North Dakota unemployment compensation law as of January first of any calendar year subsequent to such two calendar years, only if during January of such year it has filed with the bureau a written notice to that effect.