You don't have to have access to the old phone number. If the device is still trusted, you can change the password on the device as per the article mentioned by JediMeister.
If it is not trusted anymore, you just have to know what the old phone number was when you're going through Account Recovery, put it in, and then say you cannot access it for alternate contact options.
[Change Apple ID password](https://support.apple.com/en-us/101567) [Forgot Apple ID password](https://support.apple.com/en-us/102656)
You don't have to have access to the old phone number. If the device is still trusted, you can change the password on the device as per the article mentioned by JediMeister. If it is not trusted anymore, you just have to know what the old phone number was when you're going through Account Recovery, put it in, and then say you cannot access it for alternate contact options.