YouMail needs cellular data to successfully dial the activation code(s)
Since the activation codes attempt to add a forwarding number in your carrier's "Conditional Call Forwarding" setting, a strong connection is needed to make these changes. Some common error codes with poor connection are: OCN and MMI.
Wi-Fi calling, while it enhances your connection, does not supplement for the issues of poor cellular connection. Using Wi-Fi for the activation codes will result in the same errors, as cellular data is needed to make changes in your carrier settings.
If the activation codes still do not work with strong cellular connection, please contact us at 1-800-374-0013 or start a live chat with us here.