Open the Phone app
Tap Recents
Find the caller you want to mute
Tap the info icon next to the number
Scroll down and tap Block this Caller
Tap Block Contact to confirm
Open Settings
Tap Phone
Tap Silence Unknown Callers
Turn Silence Unknown Callers on
Open Settings
Tap Focus
Set up Do Not Disturb or another Focus mode
Allow calls only from selected contacts
Turn on the Focus mode when you want calls muted
Open Settings
Tap Sounds & Haptics
Tap Ringtone
Set the ringtone to None if available through a custom tone setup
Turn on Silent Mode using the Ring/Silent switch
Lower the volume using the volume buttons
Use Airplane Mode to stop all calls temporarily