Anonymous calls are calls that list private or have a blank space where the incoming number is displayed. These types of calls are used by businesses that do not want to be called back by the person receiving the call. These include telemarketers and collection agencies. This type of call is also used by pranksters to make joke, or "crank," calls. Many of these calls can be blocked from your T-Mobile phone.
Open the menu.
Click on "Block List."
Select "Unknown Number." This enables a setting to block all calls that do not list a number.
These steps are will work for most T-Mobile phones. If your phone lacks a "Block Calls" setting, you may be able to get calls blocked by contacting T-Mobile directly.