Besides the software compatibility with protocols different from GSM (2G) there is serious hardware problem: RLT-SDR dongles can't operate above 1.4 GHz. This limit is for the better models (i.e. those with a better shielding ). In my experience the cheap models as the one mentioned in the article have problems above 1 GHz. I'm telling you because GSM support ~900 Mhz and ~1800 Mhz frequencies and, as I tested, only the traffic around 900Mhz is visible. So, even in the 2G domain, you can't see everything.