Callis became a practicing physician, Howard University Professor of Medicine and 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. 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’s Moorland-Spingarn Research Center.