A Toronto-area woman was slapped with a bill for more than $45,000 after selling her cellphone online.

Oshawa resident Kelly Arsenault told CTV Toronto she thought she'd finished her mobile contract, so she sold her phone to a stranger through an online ad on Craigslist.

Arsenault thought her account was closed, but someone started using the phone, and amassed a bill of $45,585.83.

"No, this can't be right. Are you sure you have the right people?" Arsenault said she asked when she opened the bill earlier this month.

Arsenault opened an account with Telus three years ago, opting for a business account because of a promotion at the time. Her contract ran out in September, so she sold the phone to a woman she met online.

"We believed the contract was closed. We believed we had no further charges," Arsenault's sister Christine said.

The case is under investigation, but it appears that the person who bought the phone realized that Arsenault's account was still active.

The person using the phone has used tens of thousands of megabytes of data over three months, leading to the massive bill.

Telus told CTV Toronto that it is not unusual for business accounts to have large bills. The company also sent more than 120 overage notifications to the phone number.

Customers of all providers are warned to be sure their contracts are cancelled before selling or giving away their phones, Telus spokesperson Shawn Hall warned.

"If you have a phone that leaves your person, it's stolen, or you sell it or give it away, you must call your carrier promptly to make sure something like this doesn't happen."

Telus is working to track down the person using the phone, but told CTV they do not expect Arsenault to pay the bill.

With a report from CTV Toronto's Pat Foran