AT&T Inc., the second-largest U.S. wireless carrier, is planning to sell bonds in a benchmark offering to help pay back some of the $3.25 billion of outstanding debt that comes due this year.

     AT&T may issue five-, 10- and 30-year debentures as soon as today, according to a person familiar with the offering. Proceeds may be used for general corporate purposes as well as repayment of its outstanding bonds, said the person, who declined to be identified because terms aren't set. Benchmark offerings are typically at least $500 million.

     Corporate bond offerings are showing signs of life, with borrowers from Occidental Petroleum Corp. to San Diego Gas & Electric Co. joining Dallas-based AT&T in tapping the market as yields on investment-grade debt hover at about the lowest on record. Sales rose 43 percent last week to $12.1 billion after falling to the lowest since June during the period ended Aug. 5, according to data compiled by Bloomberg.