A few years ago I had a phone made by the Romanian company X-Cellular. It was a “dumb” flip phone that had numerous anti-tracking features, including: dynamic random IMEI and the ability to choose cell towers.
It would display a list of available towers with signal strength. Most phones immediately connect to the tower with the strongest signal (which is how IMEI catchers and Stingray technology works… those devices generate extremely powerful signals that mimic real towers). However, this phone allowed the user to choose towers with lower signal strength (presumably further away or at least not a Stingray device). Apparently that would spoof the location of the phone, making triangulation difficult or impossible.
The phone also alerted the user every time the phone was pinged. Pings are apparently detectable but not preventable. The only way to stop them is a faraday bag or removing the battery. However, pings did not always occur. They seemed somewhat random - several times a day. Sometimes just once a day.
The IMEI randomizer made it more difficult for Stingray devices to target the phone - although it’s not clear to me how the phone was still able to access cell networks when the IMEI number didn’t match the registered IMEI.
Finally, regarding actual silent SMS attacks with malicious payloads, iirc, the way this was handled was to max out the SMS messages stored on the phone/SIM. There were hundreds of “dummy” messages stored. I was repeatedly told to never erase them or it would make me vulnerable to stealth attacks that could install malware on the device. As long as the standard inbox was full, silent SMS messages would fail due to a kind of “mailbox is full” error. However, the phone was still somehow able to receive messages - possibly in some sort of sandboxed part of the phone that prevented malicious payloads from executing.
All of that said, it’s my understanding that the firmware on the phone was heavily modified to allow all of these features. The developers also told me that “smart phones” are nearly impossible to secure because of the numerous sensors. However, it would seem that some of the above strategies could potentially be employed…