National Grid is warning customers in St. Lawrence County and beyond of what they describe as “an ongoing utility billing scam.”

National Grid’s Customer Contact Center has received calls from residential and business customers saying people contacted them by phone who claim to be from National Grid and advise the customers they have a past due balance on their utility bill.

Similar scams have been reported across the U.S., National Grid said.

The scam involves a caller who pretends to be with National Grid (or another utility) and tells the customer that their power will be shut off immediately unless the customer purchases a pre-paid debit card in a specific amount, such as Green Dot card, and provides the caller with the card’s account number, or in the case of business customers, by way of a Western Union money transfer. The scenario can change, but the goal of the scammer remains the same: scare customers into making hasty, often large payments.

“Scammers have become increasingly sophisticated in replicating National Grid’s recorded messaging and directions for phone prompts making it more difficult to differentiate an actual National Grid call from an imposter’s call,” the utility giant said in a prepared statement. “National Grid does contact customers with past due balances by phone to offer payment options, but never demands direct payment through the use of a pre-paid debit card and never accepts payment through these cards.”

Customers who believe they have fallen victim to the scam should contact National Grid and report the scam to their local law enforcement officials and the Attorney General’s office immediately.

The company says they encourage customers to verify they are speaking with a National Grid representative. One way to do this is to have available the last five digits of their National Grid account number and ask the caller to provide those numbers.

National Grid urges customers to be cautious of scammers and offers the following tips:

• National Grid representatives will know one’s account number, so never offer that information to a caller

• Ask the caller to provide the last five digits of one’s National Grid account number. If the caller doesn’t know the account number and there is doubt the caller is a National Grid representative, or if they have any questions about account balance and fish for help, take charge and hang up immediately. Call National Grid or local law enforcement officials

• National Grid may ask for a payment over the phone, but will leave the method of payment to the customer

• National Grid will not contact customers demanding immediate payment by wire transfer, Green Dot Money-Pak or any other pre-paid card service

• Never -- under any circumstances -- offer personal or financial information to someone who cannot be identified

“Every National Grid employee carries a photo ID card, and any contractor doing work for the company is also required to carry ID. If someone requesting entry into a home or place of business does not show an ID card, don’t let that person in and call National Grid or your local law enforcement,” the company said.