Hello, unfortunately, there isn't much that can be done to recover your account if you no longer have access to the phone number connected to your account. Obviously, this is not the news that you want to here.
When you register for an account, it's up to you to provide the most up-to-date and valid recovery info. Your best option to avoid losing access is to update your recovery info if you get a new phone number or change your alternate email address.
You can have a look in this article produced by CCM giving you all of the possible solutions on how to recover a yahoo account. The article is exhaustive, containing all the known solutions. A final option would be to contact yahoo support directly.