tl;dr the sms verification falls back to voice and they just used a payphone.

I guess if you count the airport full of cameras they went to to do this as "anonymous", then sure :)

Also this article from 2017 suggests not using this method:

It’s important to maintain control of this phone number. For example, you could use a disposable SMS service to register with Signal — there are many such services if you search for them — but those phone numbers can be used by anyone. Similarly, you should avoid using a public payphone’s number, or a SIM card on which you do not intend to renew service. If someone else can receive SMS messages or phone calls to this phone number, they can take your Signal account away from you.

How I Got a Truly Anonymous XMPP Account:

• Open my client (e.g. Conversations, Monal, Dino)
• Pick a random server, username and password
• Click register

Sorry, it's a cheap joke, but it still baffles me that Signal requires a phone number, so I felt I had to post it :)

Of course, this is not XMPP-specific either, just my protocol of choice, there are many other open alternatives that also offer such functionality.