I think they are working on non-phone number IDs though (Moxie was in an earlier signal thread on HN recently and mentioned it).
It will take the authorities a bit more time (i.e. someone throws away their burner phone and authorities hack it)(with the assumption that phone numbers/SIM activations are provided using valid ID as it happens in many countries).
https://en.wikipedia.org/wiki/Global_surveillance#Infiltrati...
Edit: And that from netzpolitik (highly trusted german source) under 'A global passive adversary' that's the interesting part: https://netzpolitik.org/2017/secret-documents-reveal-german-...
There is also I2P network, which is even harder to break (unless someone owns practically all nodes there).
And no you can trace it thru the ISP's, the problem is the latency, Connection from here to there in that millisecond trace one...and so on.
Edit: GnuNet, RetroShare and ZeroNet should also be mentioned