Anonymous SIM cards make it much more difficult for state actors and other attackers to choose their targets. Whether silent text messages, IMSI catchers, inventory data information, traffic data information, radio cell evaluations, source monitoring, state trojans or location monitoring. An anonymous SIM card is often the only sensible defense against such surveillance.
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The subject of mobile phone surveillance is complex and cannot be dealt with in full within this framework. However, it is important to understand that security apps cannot do anything about such surveillance because, for example, it affects data that is already stored by your provider and not on your phone. Or because the apps themselves do not have access to your phone's proprietary radio chip and therefore cannot see silent text messages, for example. Or because the attack takes place in the wireless network between network providers. Or because your mobile provider simply sells your data on. So relying on apps or changes in behavior won't help. The only defense is anonymous SIM cards. Also bear in mind that in Germany, over 100 government agencies can access people's phone numbers and vice versa without a court order.