Detection of silent SMS

A silent SMS does not alter or allow anything on your device. It’s a flash SMS with no content. It’s only purpose is that the sender will get a notice it was sent without altering you, and they can then, with the cooperation of the cellular carrier, approximate your device’s location from the network towers. It’s sole purpose is to let the sender know a device they are interested in locating is connected to the network (and they determine your location from cell tower triangulation, not remote gps location data from your device).

But the silent SMS does not get stored on your device, it cannot install anything, alter any settings or allow any form of remote access.

And as I said, in countries where it is legal for law enforcement to use silent SMS for approx. location information, there is nothing at all you can do to stop anyone sending out a Silent SMS. If you don’t want to get them, all you can do is turn of your smart phone and not use it. That way the sender will never get the “message sent” indication to know whether you’re on the cellular network or not. But it is a purely passive method that does not, and cannot, alter anything on or about your device itself.

A silent sms is not malware, not spyware, not a hack or even computer code of any kind - it’s an empty flash SMS text that comes to your phone (thus indicating to the sender it was sent so they know your device is active on the network) and then evaporates since there’s nothing for your phone to display (so you get no notification since no message content was actually sent). There is nothing that you, as the owner of a receiving device, can do about them. It would be up to the countries you’re in to outlaw them, or the carrier’s there to decide not to propagate any and all flash SMS.

There is no way for a 'silent SMS' or anything else to be able to monitor or control an iPhone.

There are no malware or virus for a non jailbroken Apple device.

Well, first off, flexispy software has nothing whatsoever to do with Silent SMS (https://edri.org/edrigramnumber10-2silent-sms-tracking-suspects/).

Flexispy markets a software product, which if manually installed on a device (it cannot be remotely installed) can be used to monitor devices. That sort of tool is primarily used to monitor company devices that are intended solely for company related use. Most companies I have heard of that use such software make it publicly known to their employees in their formal IT use policy. Highly secure compute resources als often use such software (for example, I know that the US military has usage monitoring software on at least some hardware at many of their defense research labs, and the military and contract employees using those equipment know it is being monitored for unintended use).

A silent SMS (which is just a messageless Flash SMS) doesn’t require anything to be installed or altered on the receiving device. It is not a hack, nor malware. It is nothing more than a messageless Flash SMS so the sender can get a delivery receipt and know the recipient’s device is active, which allows someone with access to the carrier’s network to use tower triangulation to locate someone’s approximate location.

There is no way for you or anyone to block Flash SMS if the carrier’s network allows them. In the USA no carrier propagates Flash SMS - they all block them from transmitting over their networks. However, in Europe and elsewhere, Flash SMS are allowed, and in Germany a Silent SMS is legally not even a “communication”, and it is legally available to police for use in tracking people.

According to news reports, it’s also entirely possible for someone (police have done this themselves in some countries) to use mobile transmission towers to create hidden local cellular networks, so those could also be used to send a Silent SMS to someone’s devices. You would never know if one was sent to you, and there is essentially nothing you can do to stop them, especially in countries where police are legally allowed to use them.