I, too, am on Google Project fi (Nexus 5X phone). My wife's iPhone is my trusted number, and she's getting several "number changed" text each day. I drive a delivery route, so I'm moving around quite a lot each day (abt 250 miles). Project fi phones, when moving about, not only switch from one cell tower to another, they can change from one carrier network to another. I wonder if a network change is what's triggering the msgs? You might pass this on to the developers to see what they say.