I recently figured out, that the web site of my phone provider (mobile) mysteriously identifies me and automatically logs me into my account when I'm accessing with my android phone, as if it knew my phone number. (I used several browsers. When I'm using the phone as WLAN hotspot and access the same site from another device that doesn't happen.)

How does my phone provider do that? On the one hand, as a programmer, I'd like to be able to do that too, but on the other hand, as a user, I'm kind of scared. What information do they have, such that they (believe they) are able to identify me just by my device?

I hope this question isn't completely inappropriate for this site; feel free to add better tags — it's hard to find the right ones without knowing the Webmasters site at all.