A lawsuit filed last week alleging unlawful billing practices by CenturyLink has prompted a class-action suit seeking as much as $12 billion based on the telco’s nearly 6 million subscribers.

In her suit filed in Arizona Superior Court, Heidi Heiser said she was fired from her job as a CenturyLink customer service and sales agent days after notifying CEO Glen Post of the alleged scheme, according to a report by Bloomberg. Her suit alleges CenturyLink “allowed persons who had a personal incentive to add services or lines to customer accounts to falsely indicate on the CenturyLink system the approval by a customer of new lines or services,” sometimes resulting in charges that weren’t authorized by customers.

CenturyLink’s Mark Molzen

On Sunday, Geragos & Geragos filed the class-action suit in U.S. District Court for the Central District of California on behalf of Craig McLeod and Steven McCauley, two CenturyLink customers, accusing the telco of fraud, unfair competition and unjust enrichment.

The complaint said Heiser’s allegations of what she observed, and “what the CenturyLink corporate culture encouraged, are consistent with the experiences of hundreds of thousands and potentially millions of consumers who have been defrauded by CenturyLink.”

Among the allegations in the class-action suit are: billing consumers for phone lines or service items they didn’t request; billing consumers higher rates than those quoted during sales calls; billing early-termination fees when consumers canceled their service due to higher fees; billing consumers for service before it was connected and for products never received, with no credit for those charges; and billing consumers for products and services they never requested with no credit for the charges.

CenturyLink spokesman Mark Molzen said the telco “holds itself and its employees to the highest ethical standards and does not condone any type of unethical behavior.” CenturyLink is in the process of acquiring Level 3 Communications, with the $34 billion merger expected to close by Sept. 30.

“The allegations made by our former employee are completely inconsistent with our company policies, culture and unifying principles, which include honesty and integrity,” he said. “Should an employee have any concerns about ethics or compliance issues, we have an Integrity Line in place, 24 hours a day, seven days a week. This employee did not make a report to the Integrity Line and our leadership team was not aware of this matter until the lawsuit was filed. We take these allegations seriously and are diligently investigating this matter.”

Molzen said the fact that a law firm is “trying to leverage a wrongful termination suit into a putative class action lawsuit does not change our original position.”