The user's guide says to charge completely BEFORE turning on the phone for the first time. I plugged in the phone...it had a partially green battery looking photo on the screen, but after a short time, the screen went black. How do I know if it has fully charged unless I turn the power button on? Stupid question..I know. But I don't want to accidentally turn it own before it's completely charged and start thru the initial setup process on a partial charge. About how long does the initial charge take?