An anonymous SIM card is often one of the few remaining defenses against such surveillance. Without a registered telephone number, the target can only be selected with difficulty or not at all.
The topic of mobile phone surveillance is complex and cannot be dealt with in full in this context. However, it is important to understand that security apps cannot do anything against such surveillance because, for example, data is affected that is stored with your provider anyway 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 behavioral changes is of little use here. The only defense is anonymous SIM cards. Also bear in mind that over 100 government agencies in Germany can access people's phone numbers and vice versa without a court order.
You can order anonymous sim cards on the Internet. The search engine of your choice will help you. This project deliberately does not want to advertise individual services. Sometimes there is also scam. Please keep in mind that anonymous cards are probably often registered by marginalized groups like migrant women to earn some money. Many cards are backed by real people who vouch for you with their own name. Therefore, only use the card to protect yourself.