That depends on phone model / manufacturer. If it is Android based, you can go to call rejection list from call settings. However more details are required.
As previously suggested, it depends on the device and operating system.
Some operating systems have that functionality built-in, and for some others, you might need to install some third party application to be able to block numbers.
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Some operating systems have that functionality built-in, and for some others, you might need to install some third party application to be able to block numbers.