USSD is the ancient (since GSM/"0G") cell phone feature that handles special phone numbers like *123#. The upside is it's available even on the non-smart ("feature") phones. But in the US, it's practically unused, everyone does phone apps instead. Author seems to really loves this, "it’s a blueprint to build more resilient, efficient, cost-aware systems anywhere in the world."

Unfortunately author failed to mention the big problem with USSD: it's only provided by the carrier. So if you thought that getting app into the appstore is hard? USSD requires you to go to each carrier and sign contract with each. In US, I am pretty sure none of then will even talk to you unless you have millions of dollars to spend.