Turn on Silence Unknown Callers: Settings > Phone > Silence Unknown Callers
Enable carrier spam blocking or call filtering in your mobile carrier app
Use a third-party call-blocking app from the App Store
Block specific numbers: Phone app > Recents > info icon > Block this Caller
Report spam calls to your carrier
Report and block spam texts in Messages
Keep iOS updated
Do not answer or interact with robocalls
Do not press any buttons or speak to automated callers
Register your number with the National Do Not Call Registry
Check and remove suspicious call-forwarding settings
Set contacts for important callers so unknown numbers are easier to ignore