Thank you for using CEX.IO.
You might have noticed there is an ongoing issue concerning the codes that are sent to mobile devices.
Along with CEX.IO, a list of other platforms which provide 2FA (phone call/SMS option specifically) does face the same very problem as well.
We are having this issue fixed, so thank you for patiently waiting. Please be asked to check the resolution status on the following page: Status Page
SMS/Phone call delivery issue