This happens in several parts of Europe as well. It's part of the telcos' billing infrastructures, and many operators for example have middleboxes which allow TCP streams to be looked up against the billing system.

I believe the original idea was to allow companies selling ring tones to able to bill customers who downloaded their ring tones directly on the customers' telco bill.

From a privacy standpoint it's been a catastrophe. There are countless of operators who have been caught decorating customers' outgoing HTTP traffic with their mobile number or personal details. It's just a few years since one operator was caught doing this in Denmark [1].

Again, just a few years ago, in Sweden, a company setup porn sites and pretty much blackmailed their mobile visitors into paying $$$ for porn they supposedly had agreed to download. This company was using operators' billing APIs to lookup subscriber details from the IP:port numbers of connections to their porn sites [2].

In Norway, a company called MobileTech, use the same APIs to improve unreliable web tracking using cookies. By using these billing APIs they can assign a unique identifier to a particular subscriber regardless if this subscriber clears their cookies or share the connection across multiple devices. Their tracking script (b.mobiletech.no iirc) is embedded on many popular nordic sites. Their improved visitor tracking and demographic data is also sold to third party marketing companies such as Research International.

[1] https://www.version2.dk/artikel/mobilsurf-danske-teleselskab...

[2] https://www.svt.se/nyheter/lokalt/skane/fangelse-for-skaning...