Open the Phone app
Tap Recents
Find the blocked call or number if it appears in your call history
Open Settings
Tap Phone
Tap Blocked Contacts
Review the list of blocked numbers and contacts
Open Settings
Tap Messages
Tap Blocked Contacts to see blocked message senders
Open Settings
Tap FaceTime
Tap Blocked Contacts to see blocked FaceTime contacts
Use a third-party call-blocking app if you need a separate blocked-number list from that app
Check your carrier account or app if the number was blocked there instead of on the iPhone