FORT WORTH, Dec 19 (Reuters) - American Airlines, the world's largest airline, on Wednesday began recalling nearly 800 reservations representatives as call volumes to its 10 domestic reservations centers increase. The unit of AMR Corp. (AMR) said calls are taking longer to complete because of increased security processes and a change in the mix of leisure and business travelers since the Sept. 11 attacks. Another carrier, No. 4 Northwest Airlines (NWAC), also plans to recall 312 part-time telephone operators that were laid off after the Sept. 11 attacks which threw the industry into a crisis. It, too, cited longer calls due to schedule changes, fare structures and security questions. American said more customers have been calling its reservations centers in recent months than originally anticipated, sparked by low fares.