This is the best summary I could come up with:

Based on a phone number, the federal prosecutors were asking for the user’s name, address, correspondence, contacts, groups, and call records to assist with an FBI investigation.

Whenever Signal receives a properly served subpoena, they work closely with the American Civil Liberties Union to challenge and respond to it, handing over as little user data as possible.

Whittaker stressed that this is “a pretty narrow pipeline that is guarded viciously by ACLU lawyers,” just to obtain a phone number based on a username.

Signal’s leadership is aware that its critics’ most persistent complaint is the phone number requirement, and they’ll readily admit that optional usernames are only a partial fix.

She gave an example of a person who faces severe threats and normally maintains vigilance but whose mother is only on WhatsApp because she can’t figure out the numberless Signal.

Signal engineers have discussed possible alternatives to phone numbers that would maintain that friction, including paid options, but nothing is currently on their road map.

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