Most systems HAVE to mark shorteners as spam, given their use.
Can you think of reasonable and serious reason to shorten another shortener?
SMS doesn't care what kind of characters you're sending.
SMS can't click on links either, so just send the word LINK and get done with it.
You can click on a link in a text message even on my dumbphone.
Why not just put a URL shortener on the SMS interface, where it doesn't matter much if the shortener service dies anyway, and leave the original URL on the web interface.