On iPhone

Block a number manually

Open the Phone or Messages app.

Tap the number → Info (i) → Block this Caller.

Silence unknown callers

Go to Settings → Phone → turn on Silence Unknown Callers.

Calls from numbers not in your contacts go straight to voicemail.

Filter unknown senders in Messages

Go to Settings → Messages → turn on Filter Unknown Senders.

On Android (steps may vary by brand)

Block a number manually

Open the Phone app → recent calls → tap number → Block/Report Spam.

In Messages: open the text → menu (⋮) → Block & Report Spam.

Enable spam protection

In Phone app → Settings → turn on Caller ID & Spam.