Verizon Wireless has been adding a unique identifier to its cell phone customers web traffic. The identifier can be used to track a users actions in exactly the same way as a cookie but, unlike a cookie, it cant be deleted. Even if you opt out of Relevant Mobile Advertising that pesky HTTP header gets added to every request that leaves your phone and before it hits the internet. Unlike cookies, Flash Cookies, Web Storage or ETags that are normally used for tracking you can’t hide yourself by deleting it.”]
Source: https://grahamcluley.com/verizon-gives-120-million-customers-cookie-cant-delete/