The alert is real, and T-Mobile claims it's something that can impact all cell phone carriers.
Here is the text T-Mobile sent out to its customers:
This is how the scam works: Thieves have a victim's phone number, they pretend to be the consumer and transport that number to another phone - basically hijacking the phone number.
A phone number is a key that opens a lot of other doors, like bank account information and Facebook and Twitter accounts.
Once the thieves have control of the number, they're able to change the user's passwords, access their accounts, and steal their money.
T-Mobile has information on their website on what steps users can take to protect themselves.
The company is now encouraging their customers to add an extra layer of security to their phone number.
Users can create a passcode or PIN for their account. Then, they're required to provide that code before a new SIM card is issued or any other changes are made - most cell phone providers now offer this security feature.
If you're a T-Mobile user, you can call 611 from your cellphone or 1 (800) 937-8997 to set up a passcode.
Those who have another carrier should check to see if they can add a password or PIN to your account.