Where I can check the numbers that are opted out or have been blocked?

To check the numbers which have been opted out from your receiving messages you can simply use the "Blacklist" option under the "Segmento<Phonebook"

On this page, you will get the list of all the numbers which have been blocked by you or either the user has unsubscribed. 

Here contacts can be added to the blacklist and you can also export all the blacklisted numbers from this page.