Verizon (VZ) is currently facing claims that it has been overcharging customers for its landline service.

Consumer-advocacy group New Networks Institute, which focuses specifically on benefitting the telephone customer, is considering taking legal action against Verizon, the New York Post reports. The New York-based group claims that the telecommunications provider has been overcharging its landline customers by $1,000 to $1,500. With 2 million Verizon landline customers in New York state, that adds up to billions of dollars.

NNI executive director Bruce Kushnick told the Post that “copper-based phone services should be $10 or $20 [per month].” Verizon charges landline customers a basic fee of $23 per month. With taxes and optional add-ons—including unlimited local and long distance calling, voicemail, call waiting, call forwarding, and home wire maintenance—that number can shoot up to more than $60. Kushnick added as an example that call forwarding and call waiting cost Verizon just 2 or 3 cents, but customers pay $7.95.

Additionally, the NNI claims that Verizon wrongly uses the extra money for corporate expenses and its mobile and Fios fiber-optic networks, when it should instead be going towards maintaining its landline service.

This news comes as New York’s Public Service Commission has begun investigating whether Verizon properly maintains its landline service. Though the company says it’s complying which the state, it denies NNI’s claims. “We think Kushnick is wrong on a number of levels but primarily because there is absolutely no factual basis for his allegations,” a Verizon spokesperson told Fortune. “We continue to make significant investments in all our networks—copper, wireless and fiber—and we make no apology for doing so.