T-mobile customers who applied for postpaid services between Sept. 1, 2013 and Sept. 16, 2015 are affected by the breach.
Information taken from the server included customer names, addresses and Social Security numbers.
Attorney General Chris Koster issued an alert about the data breach On Wednesday, Oct. 7.
"My office is monitoring the situation to ensure the Missouri consumers are properly notified," said Koster in a press release.
Koster said consumers who are affected should consider registering for a credit freeze.
In a letter to consumers, T-Mobile CEO John Legere said he will focus on helping consumers.
"Obviously I am incredibly angry about this data breach and we will institute a thorough review of our relationship with Experian, but right now my top concern and first focus is assisting any and all consumers affected," Legere wrote in the letter in the letter.