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If you want to block a person and prevent them from calling you, android will have the option to just do that.
You first need to know if they call as a unknown caller or show their phone number.
For the last case the solution is easiest.
-If you didn't already, add this number to your contact list.
- go to that contact and click block this contact
- my device asks if I want to enable anti harassment, in this case I choose yes, but this can differ from device to device.
- after completing these steps the caller will not be able to contact you anymore and will go directly to your voicemail.
if the caller uses a unknown caller ID, you do have the option to block all unknown callers, however also companies use this option often and they will not be able to call you anymore as well.
If you want to do this you can use the app Mr. Number-Block calls & spam