For AT&T home phone (VoIP) users:
AT&T Call Protect is a free network-level spam and block management tool. To unblock a number: att unblock phone number
If you blocked a number directly from your iPhone’s Phone or Messages app, here’s how to undo it: For AT&T home phone (VoIP) users: AT&T Call
For most Android and iPhone devices, unblocking is managed directly through the built-in phone app settings. on your device. Tap the Menu icon (often three dots or "Settings"). Select Settings and then Blocked numbers . att unblock phone number
You can also manage blocks online via your AT&T account under → Call Features → Selective Call Blocking .