It might be one of those scenarios based on a pattern of behavior that Google is in a position to identify and an individual user is not because the patterns include temporal, spatial, network, etc. elements that emerge at the scale of many users and across multiple web properties.
I'd put it this way, if Google wants your phone number it can just Google it...or rather, Google already has your phone number and a whole lot more because its ordinary data and information leaks everywhere across the web. Trivially, all it takes is for anyone who has ever had your phone number to have shared their address book online and odds are Google has your name and number.
I'm not saying this is good (or bad). Just that it is. In the moment by moment, a person can kind of avoid some tracking because the speed, volume, and economics at which advertising is auctioned limits the depth of historical search on a moment by moment basis. But when it comes to a specific ordinary fact, Google has it already.
The fraud detection could in theory be triggered by something as obscure as the angle of your phone and its acceleration parameters being unusual.