You need to call customer support for your phone... this will vary depending on the area you live in... at this point I know Virgin Mobile and Sprint both carry them (NY and CA)
They will be able to tell you if it is a known network problem or if it is a problem with your phone.
Make sure you call from another phone because they will walk you through the steps to try and fix your problem.. most likely they will first do a soft reset of your phone.