We looked at this issue back in 2013, because it appeared we should be able to successfully use things outside of 7-bit ASCII. However, Sierra Wireless told us that their modems “probably don’t support that”, and we couldn’t get any further support on the issue from them. We eventually gave up trying to make it work. :\
You might look into the Twilio module instead. Twilio’s service offers full Unicode support, so your 8-bit ASCII will work as well.