Turn on Silence Unknown Callers: Settings > Phone > Silence Unknown Callers
Enable carrier spam protection features in your mobile carrier app
Block individual spam numbers: Phone app > Recents > info icon > Block this Caller
Report spam calls to your carrier
Use a call-blocking app from the App Store
Turn on Do Not Disturb or Focus mode and allow only contacts
Filter unknown callers to voicemail by using Silence Unknown Callers
Keep iPhone updated to the latest iOS version
Avoid answering or calling back unknown numbers
Remove your number from marketing lists and data broker sites
Register your number on the National Do Not Call Registry
Check and disable call forwarding if it is enabled
Reset network settings if call issues continue: Settings > General > Transfer or Reset iPhone > Reset > Reset Network Settings