How do I add custom MPie S168 Text Tones?

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AC Question

MPie S168 Text Tones

Does anybody know how to add custom text tones to the S168 from MPIE. I have managed to update my ringing tone but cannot find how you can add a new text tone. I have seen you can add a tone for certain contacts but I just want one for any text that comes in rather than the generic tone on the phone. Thanks