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Those little charges on your phone bill can add up - but do you know what they're for?

The Federal Trade Commission announced Tuesday that it was suing T-Mobile US. The mobile provider is accused of billing customers for services they never authorized, such as celebrity gossip and horoscope texts.

T-Mobile cast blame on third party vendors for the charges and in a statement said the allegations are "unfounded and without merit," according to the Associated Press.

The mobile provider said it is already offering refunds.

The practice of billing customers for bogus charges is called cramming and involves tacking on charges both small and large that appear legitimate but aren't.

The FTC offers these tips to make sure you're not being scammed on your phone bill:

• Don't enter your mobile phone number on unsecured websites.
• Unsolicited text messages can be a sign you're being billed for a service you didn't opt in to. Look closely at your next bill to make sure there are no new charges.
• Speaking of which: The most common cramming charge is $9.99, a small amount that is easy to overlook. If you see a charge and don't know what it's for, ask your mobile provider. "Your carrier should be able to tell you more about the charge, and your statement should tell you how to dispute errors on your bill," the FTC said.
• Look out for generic sounding fees. Some common words that may be used include: Min. Use Fee, Activation, Member Fee or Subscription.
• Some carriers offer a feature that will block third-party services on your phone bill. If you're worried about bogus charges and don't plan to use third party services, ask to opt-in to the service.