As of 12 a.m. today, eastern Massachusetts residents will have to dial a full ten digits in order to make local phone calls.

Residents in Greater Boston's 617, 781, 978 and 508 area codes will make the transition to ten digits, a move that is affecting an estimated 4.3 million phone lines.

In addition to having to dial extra digits, the change will require reprogramming telephone and fax speed dials, modems and computerized telephone equipment used in business offices.

While officials say that some phone numbers may still be able to be dialed using seven digits even after today's change, Verizon Communications should have all necessary upgrades completed by mid-April.

"We hope and expect it's going to be relatively painless, but there may be some stumbling around for a couple of days," Timothy Shevlin, executive director of the Massachusetts Department of Telecommunications and Energy, told The Boston Globe last week.

The ten-digit method has been available for use since last September, allowing a seven-month adjustment period before today's mandatory change.

After today, those who call numbers without dialing an area code first will receive a pre-recorded message saying, "To complete your call, you must dial all 10 digits, including the area code of the number you are calling. Please redial using the complete 10-digit number."

The changes are occurring partly because of the explosion in computer, fax and cellular phone lines. In addition, the deregulation of the phone industry has created problems-over 50

local phone carriers operate in Massachusetts, and until recently, the Federal Communications Commission required the state to distribute phone numbers to companies in blocks of 10,000, meaning many phone numbers went unused.