state regulators or consumer groups. However, overlay plans were eventually used much more widely in some areas than others. For example, much of Ohio has large overlay complexes, as has northern Georgia and the North Carolina Piedmont. Connecticut, Illinois, Oregon, New Jersey, Massachusetts, Maryland, Pennsylvania, Texas, Virginia and, more recently, California have also implemented many overlays. No area code splits have been performed since 2007, when area code 575 split off 505 in New Mexico, and no splits are proposed. Telecommunications companies have increasingly favored overlays even in sparsely-populated rural areas in which ten-digit local numbers are unnecessary, as split plans force cellular providers to reprogram millions of client handsets. Customers also incur costs to publish new letterheads and to reprogram stored address book data on individual devices. Overlays have become more expedient as the proliferation of cell phones has accelerated area codes exhaustion. In most relief planning projects, overlays have become the sole option considered in the planning stage. This is especially practiced for area codes that have been pushed back to the brink of exhaustion after being recently split, as carriers want to minimize additional customer inconveniences. For example, Maryland had been served solely by area code 301 from the implementation of the area code system until 1990, when everything from Baltimore eastward was split off as area code 410. Within only five years, 410 was already on the brink of exhaustion due to Baltimore's rapid growth and the proliferation of cell phones, pagers, fax machines and dial-up Internet lines. However, the area's dominant telephone provider, Bell Atlantic (now part of Verizon) realized that a split would have forced residents of either Baltimore or the Eastern Shore to change their numbers for the second time in the 1990s. Accordingly, area code 443 was created as an overlay for all of the eastern portion of Maryland in 1996. Overlay plans also favor incumbent wireline carriers over new entrants, as the established firm already has large allocations of numbers in the more desirable existing code, and subscribers of new or growing competing carriers are relegated to unfamiliar, new codes. The first example of an area code overlaid on an entire U.S. jurisdiction was Puerto Rico, with area code 939 added to the numbering plan area of area code 787, in 2001. Puerto Rico had just been split from area code 809, the area code for much of the Caribbean, in