A few years ago Verizon included an XID variable in the headers (essentially a unique subscriber code), but I'm not sure what the SOP is today. Granted, there

A few years ago Verizon included an XID variable in the headers (essentially a unique subscriber code), but I'm not sure what the SOP is today. Granted, there are still several ad-testing companies that store a variety of mobile cookies for tracking impressions, but this is much different from permanent, header-based tracking that used to occur with Verizon and possibly others.