Glendale Water & Power is warning customers of a recent increase in telephone scams aimed at bilking them of thousands of dollars for phony past-due bills.

Customers are contacted by presumed customer service representatives and told their accounts are delinquent and they must pay immediately over the phone or face service shutoff. They are instructed to purchase a Green Pack Money Card — a specific prepaid debit card — from a 7-Eleven or Rite Aid and then call back to pay the bill using the card.

In the past week, three separate customers have paid the scammers between $1,000 and $2,000 each, according to GWP spokeswoman Atineh Haroutunian.

The scheme is run mostly from outside the United States, and organizers use software to make the calls appear on caller ID to be coming from GWP or from local numbers with an 818 area code, the utility said in a statement.

GWP says its representatives will never ask for payment over the phone, and customers who receive these types of calls are encouraged to contact the utility at 818-548-3300 to check the true status of their account and report the attempted fraud.

Customers are also asked to call the Glendale Police Department at 818-548-4911 to report the calls.