Go here: https://supportforums.cisco.com/blog/152066
You will need a line card/module to support x.25. Go here: http://www.cisco.com/c/en/us/products/collateral/interfaces-modules/1800-2800-3800-series-16-port-async-high-speed-wan-interface-card/product_data_sheet0900aecd80274416.html
Geez. x.25. Takes me back.