After you address all the possible causes in DOC-2726 and still have problems, you confront the real issue. This phone is horrible. It is the worst "smart" phone I've had. It makes me look back fondly at my iPhone 3g, and I wasn't very happy with it. The most frustrating issue is when I hit the "Phone" icon, then try switching to "Call Logs" so I can look up a recent number, and the app immediately shuts down. It does this repeatedly, even if you clear the memory. I usually have to reboot the phone to fix it. If I am in a hurry, it becomes an unacceptable situation. The map app locks up frequently also, which almost always means removing the battery. These are applications that t-mobile and samsung say I must have. Not third party apps. And it isn't just my phone, I bought one of these POS's for my wife too.
While we're at it, let's talk about the messaging app that crashes all the time too, and text messages that arrive 5 hours after they're sent, and the malfunctioning tower by my house it took two months for them to repair. I regret switching to t-mobile every day, and would leave if I didn't have to pay the ETF. But I have stopped several friends from signing up by relating my experiences to them.