Southwestern Bell Corp has found favour with Chicago credit rating agency Duff & Phelps, which raised the rating of the notes and debentures of the San Antonio company’s Southwestern Bell Telephone Co to Double A Minus from Single A Plus. At December 31, Southwestern Bell Telephone had $4,700m of notes and debentures outstanding. Duff said as a result of good underlying volume and aggressive cost control, it generated strong improvement in quantitative measures of credit quality in recent quarters.