Open Settings
Tap your name at the top
Tap Name, Phone Numbers, Email
Tap Edit
Tap the red minus next to the phone number
Tap Delete
Tap Done
If the number is linked to iMessage or FaceTime, go to Settings > Messages > Send & Receive and remove it
Go to Settings > FaceTime and remove it there too
If the number is on your Apple ID account, sign in to appleid.apple.com and remove it from your account details
If the number is the only trusted number on the account, add a new trusted number first before removing it