Hackers have figured out how to turn a feature phone into a “jammer” with just a few software modifications. The hack was developed by a security research group at the Technical University of Berlin. The jammers only work on 2G GSM networks, and can only block calls and texts to phones of the same provider. The technique also can’t touch 3G or 4G data, but for the time being both T-Mobile and AT&T rely on blockable GSM for texts and calls.”]
Source: https://gizmodo.com/software-hack-lets-feature-phones-jam-calls-texts-with-1201013045