Area code 978

Area code 978 was created as a split from area code 508 on September 1, 1997 and covers northcentral & most of northeastern Massachusetts (LATA code 128). Use of 978 became mandatory February 1, 1998. Prior to when this area was served by 508 (July 1988), It was served only by the 617 area code, along with the rest of the eastern two-thirds of the state. 351 has been sharing the service area since May 2, 2001. Since then, 10 digit local dialing is mandatory.

Some mobile phone numbers from the 1990s assigned to rate centers in 978 and 351 kept the 508 area code just on those mobile lines after it was split.

...
Wikipedia