If you're receiving unwanted calls or texts and you're a T-Mobile subscriber then you can block the offending number by calling customer service at 1-800-937-8997 and they should be able to block the call on their end. Some phones have the ability to block calls from the handset (Memoir™,Behold™, and Highlight™) but if you're unsure that your phone has this capability then you can call customer service and get them to walk you through the process. An alternative method or last resort is to use the Silent Ringtone Method.
Silent Ringtone Method - block any number on any phone with a silent ringtone
To block unwanted numbers with the silent ringtone method, add the offending number to your address book as a contact (you can give them a nasty name!) then assign that name a special silent ringtone. For this method you have to know how to add ringtones to your phone and also get hold of a silent ringtone that is compatible with your phone.