Callis became a practicing physician, a Howard University Professor of Medicine, and a prolific contributor to medical journals.
Often regarded as the “philosopher of the founders” and a moving force in the Fraternity’s development, he was the only one of the “Cornell Seven” to become General President of the Fraternity. Prior to moving to Washington, D.C., he was a medical consultant to the Veterans Hospital in Tuskegee, Alabama.
Upon his death in 1974, at age 87, the Fraternity entered a time without any living Jewels. His papers were donated to Howard University’s Moorland-Spingarn Research Center.