Thanks! So the paper that the other commenter linked is a good deep dive. The basic answer to your question is that we have multiple variants but the simplest is that phones using PGPP would have the same IMSI; once you do that, you hide who's who but create a new problem of how to make sure users are valid paying subscribers. We solve that new problem by developing a new oblivious authentication protocol that can verify someone is a valid user (i.e. someone who has authentication tokens that are not linked to them but are issued by the network).