Anything West of Quabbin Reservoir, which correlates pretty strongly to 413 area code. New Salem might be right on the line of 978/508/413, but let's be honest, of the 50 people who live there there's only like 10 have phones.

I think this map gets it pretty right:

https://www.orangesmile.com/common/img_city_maps/massachusetts-map-1.jpg