Area codes 610, 484, and 835 are telephone area codes in the North American Numbering Plan (NANP) for the eastern and southeastern regions of Pennsylvania. The numbering plan area (NPA) includes regions to the west of Philadelphia and the cities Allentown, Bethlehem, and Reading. It includes much of the Delaware Valley, including almost all of Delaware County, most of the Philadelphia Main Line, and all of the Lehigh Valley.

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Area code 610 was assigned on January 8, 1994, in an area code split from numbering plan area 215, which had been the entire southeast quadrant of Pennsylvania since 1947. Permissive dialing of both 215 and 610 continued until the morning of January 7, 1995.^[1] It was Pennsylvania's first new area code since the definition of the area code system in 1947.

Three exchanges which would have switched to 610 were instead switched to 717, the area code for most of the eastern half of the state outside of the lower Delaware and Lehigh Valleys. The central office prefixes were 267 at Denver, 445 at Terre Hill, and 484 at Adamstown with 267 being renumbered as 717-336 because 717-267 was already in use at Chambersburg. These exchanges were all served by independent, non-Bell telephone companies, which sought to consolidate their eastern Pennsylvania customers into one area code and would have had to change area codes regardless.

This was intended as a long-term solution, but further growth in the region over the subsequent five years, and the proliferation of cell phones and pagers, spurred the introduction of area code 484 as an overlay for the 610 region on June 5, 1999, along with the introduction of mandatory ten-digit dialing.^[2]

Area code 835 was intended as an additional code for the 610/484 overlay, as announced by the Pennsylvania Public Utility Commission in May 2000, and to be implemented in 2001. However, newly developed, more efficient number pooling measures were introduced instead, eliminating the immediate need for the new area code. The Commission formally withdrew plans for the new code on June 23, 2005, although the code remained reserved for later use within Pennsylvania if necessary.^[3] Area code 835 was eventually approved by the Public Utility Commission on December 2, 2021. Assignment of central office codes in the new area code began on September 2, 2022.^[4]

When area codes were introduced in 1947, their middle digit was either 0 or 1. In 1994, area code 610 was the last area code assigned with digit 1 in that position. The next new area codes assigned in 1995 (334 in Alabama, and 360 in Washington state) were the first interchangeable area codes.

In 1962, AT&T assigned NPA 610 for conversion to dial service of the Teletypewriter Exchange Service in Canada.^[5][6] While Telex II use of area codes was terminated in 1981 in the US by Western Union, Canadian use of 610 continued until October 31, 1993, when it was exchanged in a flash cut for area code 600 for non-geographic applications.^[7]

The following counties and municipalities are in the numbering plan area:

• List of North American Numbering Plan area codes
• List of Pennsylvania area codes