How can I sign out of an old Apple account without its inactive phone number?

Start Apple’s Account Recovery process using the account’s trusted phone number, even though it’s inactive. When prompted for a verification code, choose the option indicating that you no longer have access to that number. You’ll need to know the number and provide whatever other verification details Apple requests. Recovery can take time, and having the email address or payment information may not be enough by itself.

There isn’t a reliable way to bypass the account lock without verifying ownership. If Account Recovery fails, your dad or the organization that owned the iPad will need to work with Apple and provide proof of ownership. Avoid trying unofficial unlocking tools, since they can’t legitimately remove the account and may compromise the device.

You can check the status of the recovery process through Apple’s official account-recovery page. After recovery is approved, you should be able to reset the password and update the trusted phone number, then sign out of the iPad. If the device belonged to a workplace, it may also be managed by the organization, in which case the former employer or administrator may need to remove it from their device-management system.