Open the Phone app
Go to Recents or Contacts
Tap the number
Tap Block, Block this Caller, or More
Confirm the block
On Android, open the Phone app
Tap the number or three dots
Select Block number or Block/report spam
Confirm
On iPhone, open Settings
Tap Phone
Tap Blocked Contacts
Tap Add New
Select the contact or number
To unblock, open the blocked list
Remove the number from the blocked list