Full answer
Blocking a number prevents calls, texts, and FaceTime or messaging app contact from that number reaching your phone. The blocked caller hears a single ring before voicemail (or nothing at all depending on your carrier), and they are not notified they've been blocked. Blocked text messages are silently discarded.
On iPhone: open the Phone app → Recents, tap the blue "i" icon next to the number, scroll to the very bottom, and tap "Block this Caller." Confirm. To manage your block list later, go to Settings → Phone → Blocked Contacts. The same block applies to FaceTime and iMessage automatically.
On Android: open the Phone app → Recent Calls, long-press the number you want to block, and select "Block/report spam." Some Android skins (Samsung One UI, Pixel) label this slightly differently but the long-press or three-dot menu always reveals the option. You can manage the block list under Phone → Settings → Blocked Numbers.
For persistent spam callers, your carrier offers an additional layer. AT&T, Verizon, and T-Mobile all have free call-blocking apps (AT&T ActiveArmor, Verizon Call Filter, T-Mobile Scam Shield) that use AI to flag and block likely spam before it rings. The FCC also maintains a National Do Not Call Registry at donotcall.gov, though it doesn't stop illegal robocallers.
Third-party apps like Hiya or Nomorobo integrate with your phone's caller-ID system to identify and silence suspected spam numbers automatically, operating as a broader complement to manual blocking.