T-Mobile says it is probing a leak of its data after someone took to an online forum offering to sell the personal information of cellphone users.

The company said Monday it has confirmed there was unauthorized access to “some T-Mobile data” but was still determining the scope of the breach and who was affected.

It also said it was confident it has closed the entry point used to gain access.

Vice’s Motherboard reported Sunday someone was offering to sell personal data from more than 100 million people.

T-Mobile hasn’t said how many customers were affected.