In its fourth major acquisition this year, Los Angeles-based SmarTalk Teleservices Inc. has agreed to buy Frontier Corp.'s retail prepaid phone card business for $35 million in cash. Rochester, N.Y.-based Frontier, the nation's fifth-largest long-distance company, distributes its prepaid cards in 4,000 retail outlets, including King Soopers supermarkets, Qwik Shops and Wegmans. SmarTalk makes and distributes prepaid calling cards to major retailers, including Ralphs and Office Depot. The deal would expand SmarTalk's distribution network to about 40,000 locations. The companies expect to finalize the deal by the end of the year.