I bought an unlocked Motorola MOTOFONE F3 for about $20 at Amazon.com. With a prepaid plan, there isn’t a contract or monthly charge, just refills every now and then.
The GSM (850/1900 MHz) MOTOFONE F3 is a basic phone that seems to be targeted at the third world market. It’s good for calling someone; don’t expect to do too much beyond that and you won’t be disappointed. I wouldn’t even use it for text messaging, though it has that capability. (Maybe if you just text, “Hi.”) It’s easier just to call the other person.
The phone has an E Ink display (Electrophoretic Display – EPD) – the same type as used in the Amazon Kindle. With only two lines and six characters per line, the F3 doesn’t even display a seven digit phone number on a single line. The display is very readable in bright sunlight and it has a backlight for the keypad and screen in low light.
The F3 also has voice prompts, an address book, 500 minutes of continuous talk time and up to 300 hours of stand-by time. For $20, the thin and rugged MOTOFONE F3 is a great deal. When you don’t want to risk trashing your Android or iPhone, take the SIM out and put it in your F3.
Wikipedia has a list of undocumented codes.