Nitpick: triangulation is already very difficult or impossible on cellular networks. It requires a connection to at least 2 towers with extremely precise timing.
It’s also not required to locate your phone: each antenna covers only a certain area, which is a couple hundred meters in radius in cities. Locating is preventable by turning off your cellular modem.
IMEI randomization does not affect the IMSI, which is how the network knows who to bill for the calls.
I’m highly dubious that a phone was able to receive overt messages in some special memory dedicated to it, but would not be able to process other kinds of messages.