This is national grid. It is the same number that called me in the past when my bill was overdue. That wasn't a scam; my bill really was overdue. But I still didn't pay it by phone, I simply paid it electronically the way I pay all my bills, once I had the money to do so.
Currently, they are calling to schedule replacing my meter free of charge. This is because Mass state law requires the replacement every 7 years, even if the meter is fully functional. I converted from oil to gas seven years ago. Scam? No, not in my case.
But I'm glad so many others are suspicious in general. That is a healthy skepticism. For me, it checks out in this case.
I've received 6 of these calls in the past 4 months. 6 meters were successfully changed and they needed a person over 18 on the premises to make sure all gas appliances were firing correctly. Took no more than 1/2 ea. and they were there when promised. They change them every 5-7 years because they are made of metal that can rot out and leak gas.
I got a call from this number today -- was told my gas meter is due for replacement. I called National Grid 1800-233-5325 right away and confirmed that it's not a scam.