Twilio was hacked earlier this month, and the phone numbers of 1,900 Signal users were exposed:

Here’s what our users need to know:

• All users can rest assured that their message history, contact lists, profile information, whom they’d blocked, and other personal data remain private and secure and were not affected.
• For about 1,900 users, an attacker could have attempted to re-register their number to another device or learned that their number was registered to Signal. This attack has since been shut down by Twilio. 1,900 users is a very small percentage of Signal’s total users, meaning that most were not affected.

We are notifying these 1,900 users directly, and prompting them to re-register Signal on their devices.

If you were not notified, don’t worry about it. But it does bring up the old question: Why does Signal require a phone number to use? It doesn’t have to be that way.