To prevent unknown or telemarketers calls, iOS 13 has a feature that will allow you to block all unknown callers in one fell swoop, this is an in-built option in iOS 13 OS.
According to Apple, with this setting turned on, iOS 13 will use Siri’s intelligence to allow calls to ring your phone from numbers found in the Contacts, Mail, and Messages apps on your device — so in, other words, calls of people who you’ve interacted with in the past will still come through, even if you don’t have their number saved. All other calls will automatically be sent to your voicemail.
Here How to blocks calls from unknown callers on your iPhone running on iOS 13?
Follow below mentioned steps to block unwanted spam calls:
Step 1: Open the Setting App and Goto Phone.
Step 2: Now look for Call Silencing and Blocked Contacts.
Step 3: Under this, enable Unknown Callers.
Now all unknown telemarketing calls are blocked in your iPhone.