Los Angeles-based SmarTalk TeleServices Inc., one of the largest providers of prepaid calling cards in the U.S., said it will buy American Express Co.'s American Express Telecom unit as part of a $44-million cash transaction. American Express Telecom, based in Salt Lake City, sells its prepaid long-distance phone cards at more than 14,000 locations. SmarTalk said the transaction includes a joint marketing and technology agreement with American Express under which it will be the only provider of a co-branded, prepaid calling card that will be marketed to some American Express cardholders. SmarTalk is also expected to complete its acquisition of ConQuest Telecommunications Services Corp. next week, making it the largest U.S. provider of prepaid calling cards. SmarTalk shares fell 13 cents to close at $22.25 on Nasdaq, while New York-based American Express shares rose 81 cents to close at $87.75 on the New York Stock Exchange.