As mentioned signal doesn't hide contact discovery. But it does a pretty good job of hiding who you are chatting to from everyone but OWS.

OWS received a Grand jury subpoena and was only able to produce "the only information we can produce in response to a request like this is the date and time a user registered with Signal and the last date of a user's connectivity to the Signal service.".

Certainly a NSL might compel OWS to add additional logging (and not talk about it). With that they could tell who messaged who, when the message was sent, and how big the message was.