Open the Phone app
Go to Recent calls or Call history
Tap the number or contact you want to block
Select Block number, Block contact, or a similar option
Confirm the block if prompted
On iPhone, open the contact or recent call, tap Info, then Block this Caller
On Android, open the contact or recent call, tap More or the three dots, then Block number
To block unknown or private callers, enable Silence Unknown Callers or a similar setting in Phone settings
To unblock a number, open blocked contacts or blocked numbers in Phone settings and remove it