NStar is warning customers of a growing phone scam.

Representatives of the utility company say potential thieves have been calling customers, posing as NStar workers and telling the customer their electricity will be shut off if they don't make a payment within one to four hours. The caller then tells the customer they can only pay using a Green Dot pre-paid card available at local pharmacies and convenience stores. A callback number is provided and the scammer then walks the customer through the payment process.

NStar representatives say if customers recieve such a call, they should notify NStar by calling 1-800-592-2000.