The reason why some older mobile phones struggle with SMS containing emojis instead of just displaying tofus in place of unsupported characters is that there's no way to send emojis in accordance to SMS standard - it defines the encoding to be UCS-2. In order to put emojis in SMS, newer phones send the messages as UTF-16 instead, technically violating the standard, which can break some parsers that only expect UCS-2 to be there.