 The original TWX area codes were 510 in the United States and 610 in Canada.The addition of 710 in the Northeast, 810 in the South (plus Michigan, Ohio, Indiana, and Kentucky) and 910 west of the Mississippi allowed each major city one or more local exchange prefixes in the special area code.The special U.S. TWX area codes (510, 710, 810, 910) were decommissioned in 1981.