How does the Interceptor site define an MTS user?

Question

User identification

Mobile Internet users, when going to the site interceptor.mts.ru, can see their remaining traffic, their number and tariff.
It is curious how such an identification occurs.
There was an idea that during the transition, the external ip is compared and they look at their connections who exactly got to them. Plus, they compare the headers and remote_port (there are not enough addresses for everyone)
Does anyone know?

Answer 1

It is not difficult for the provider to find out that you are sitting with his own IP.

Answer 2

Somehow observe the issuance of IP when connecting.
Everything is elementary. From the pool they assign, give out the address - here are the determinants of you even before the thought of going to the "Interceptor")
Well, of course, this is primitively told. In fact, everything is much smarter there.