No offense, but they likely have those phones because they fit their needs. Not everyone needs or wants color screens, cameras, MP3 ringtones, and Internet access.
So the best way to get them to buy new phones is to offer them phones that have only the features they want.
Umm I'm a city boy and the only thing I want is the very phone you describe because IMHO they seem to work far better. Maybe those farmers aren't so dumb after all!