Residents and businesses on L.A.’s Westside and in the South Bay will have to dial 11 digits to make local calls starting next July as the state Public Utilities Commission on Thursday unanimously approved a new overlay area code for the 310 territory.

Under the state’s first new area code overlay, virtually all new phone numbers within the 310 territory will be assigned the new 424 area code, starting in August 2006. Existing phone numbers will keep the 310 area code. But everyone in the 310 territory will have to dial a “1” plus the area code before dialing a seven-digit local phone number.

The PUC’s decision came after a contentious hearing and capped years of bitter debate over whether the 310 area code was actually running out of phone numbers. Phone carriers had argued that the rapid spread of cell phones, pagers, faxes, credit card scanners and automated teller machines depleted the number supply. But businesses and consumer activists on the Westside alleged phone companies were hoarding unused blocks of phone numbers.

Back in 2000, after fierce opposition, the PUC shelved a plan to split the 310 area code geographically and assign the 424 area code to the South Bay. Instead, the commission ordered phone companies to give up more unused phone numbers and adopt other number conservation strategies.

Thursday’s vote came one day after the Federal Communications Commission approved the creation of an overlay only for multi-line faxes and automated teller and credit card-scanning machines. Opponents of the wider 310 area code split had urged the PUC to hold off while a technology overlay was implemented.

But Thursday, PUC president Michael Peevey said the commission had put off the inevitable split of 310 as long as possible. “We have made every effort possible to avoid a split or overlay and for quite some time our number-conservation efforts held off the need for implementation of a relief plan,” he said.