It's a bit of work, but what I've done in the past is to set the default ringtone to a silent ring tone (or the mosquito one if you're old enough and want to an

It's a bit of work, but what I've done in the past is to set the default ringtone to a silent ring tone (or the mosquito one if you're old enough and want to annoying your kids), then for each of your contacts set a custom ringtone. Doesn't scale well if you've got a lot of contacts to deal with.