the same app is actually recommended by Nokia Siemens Networks for unearthing various GSM network security vulns – its also used by the Communications Ministry in some countries as its a lot cheaper than a Rhode & Schwarz or even a SAGEM service monitor.
It does require a rooted phone with a particular chipset and the user needs some knowledge of both radio communications and the SS7 telecoms protocol to make effective use of the data they find.
Perhaps unsurprisingly it was some German hackers who developed it around 2014.
There are loads of papers about it online; most are in English but fairly technical) and aimed at telecoms engineers mostly in the developing world where SS7 vulns are more often exploited due to political issues between neighbouring countries and/or to send spam / other frauds or to disrupt a GSM telecoms network rather than to check directly where people are.
Maybe this also explains why German tech sites also mention a big revival in the use of 27 MHz CB radio across DE in the last couple of years (the BNetZA also finally allow the use of other frequencies, data transmissions and SSB)
