SMS verification codes
Why two step verification codes are successfully forwarded to my iPad but do not appear on my iPhone?None of these short format numbers are on the blocked list...
Question marked as
The older 2 step verification (4 digit codes)? -> Two-step verification for Apple ID - Apple Support
Or, the new (6 digit) 2 factor authentication codes? -> Two-factor authentication for Apple ID - Apple Support
2 factor authentication codes are not sent by SMS text. By default, they are sent by encrypted iCloud notification to all your trusted devices. A trusted devices is one where your AppleID is currently signed into iCloud and thus is tied to your iCloud account. Encrypted iCloud notifications are more secure than SMS texts.
A 2FA code would only be sent to you via SMS if you failed to get one on a trusted device (e.g. do not have a trusted device with you) and you specifically request a code be sent to your backup SMS telephone number (or backup voice number).