I think the way someone trying to steal your money would do it is simply send the message requests and when the app calls back, try to somehow hack the secretive info that's necessary to authorize the transaction.
They could hook up some automative replier that uses an algorithm to produce various sounds to try approximating the code words. What worries me a bit is that the code words would be transmitted through speech and speech through a phone call can be unclear, the software can easily mistake certain words for other ones (this could be seen by watching automated subtitle generations of youtube videos) this increases the chances for hacking the words by producing random sounds.
Maybe it would be better to send the text message to the recipient's phone which requests to type the code words and send them back?
The hackers could also hook up software that would send the same message many times (after failed attempts) and produce different words simply to try matching the code words by chance. Although the possibility of guessing the words by chance is too small, this could drain your phone's battery.
The wallet app should block the same number from sending requests after, say, three failed attempts to authorize transactions requested from that phone number.
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