[cowsup]

No. When you sign up to Signal, they send you a text message verification code. This is done via a service called "Twilio."

The attackers were able to view outgoing Twilio messages, so they could enter your number on the registration screen, read the code that Twilio sent to you, then use that cod to complete the sign up process.

Attackers were not able to view information about your current Signal account (if present) through this SMS service.

[LtWorf]

But attackers are able to impersonate you to your contacts no?

[tialaramex]

If an attacker successfully registered your phone number, and if your contact either never checked the Safety Number for your conversation† or they ignore the fact that Signal says the number has changed, then yes, the attacker would be able to impersonate you to that contact.

Twilio says 1900 Signal users are potentially affected (attackers saw the confirmation code or could have seen the confirmation code) so Signal disabled those accounts pending re-registration.

† For any particular conversation pair, Signal has a large unique number it calls a Safety Number calculated from the long term cryptographic identities, if you got a new phone (or if I'm pretending to be you on a new phone), messages you send will have a different number because the new phone won't know the old phone's cryptographic keys. The phone app can display the number (so you can compare them) or scan a QR code from another phone to check they match without the boring work of comparing numbers.