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What type of phone do you have? If you have an iPhone, go to Settings>Phone>Blocked>Add New or in Recent callers, click the i next to the number and select Block this Caller.
For Androids, the menu will vary from a phone to another, but you should find it under your contact list, settings>blocked calls or call reject.
You can also download third party apps
To my knowledge, this phone (and most flip phones) do not have the ability to block callers. There is a manual here.