I do know for a fact GSM can be monitored, but it's a very complicated endeavor. It requires some hardware purchased on eBay and the Asterisk VOIP softwareamong other software that's Linux-based. But the issue with that is you need to be near your target to trick the cell into using your little cell site, and I guess with GSM, it will happily comply with your request for plain text over encryption.
So the sentence was, "no, you can't decode it." But you actually can given the skill set and funds.
Of course trhis is illegal. So it's no, you can't monitor it because it's illegal.