Available options may vary by Phone app, dialer type, and software version. Follow the relevant steps below for your device.
Google Dialer：
Google dialer supports block calls from unidentified callers, you can enter the interface of Phone, click the three-dot button on the upper right and choose Settings--Blocked numbers--click “Unknown” to block calls from unidentified callers.
vivo Dialer：
You can long press the phone number in Phone interface--click More--Add to blacklist to add it to the blacklist. If you want to reject all strange numbers: For Funtouch OS 11.0 and higher version, enter the interface of Phone, click the three-dot button on the upper right and choose Block harassing calls--Block all unfamiliar incoming calls.
For Funtouch OS 9.0 and higher version, go to Settings--(Applications and Permissions--)System app settings--Phone--Block harassing calls, turn on Block all unfamiliar incoming calls & Block unknown/null incoming call numbers.