What do you mean by completely mobile? You can't access your account via web browser?
Or is it they locked your account and now they want you to only access using their app? If so, What is the need of a phone number? Is it for verification like they'll send a code and you'll enter it? If it is, you can hire someone for that (here on forum). Furthermore, Are you sure they're not asking for a past phone number (If you had any)?
For the app access, you can just use an android emulator
He probably think that not all features are available only on web aplication. I've noticed that recently too. For example if you want to send some funds from your wallet and you are using web aplication, sending funds has to be verified through mobile phone app. And sometimes for accessing web account you also need 2FA verfication with the mobile app.
So, actualy it's needed to combine both applications, web and mobile, otherwise you can't have full access to your account. I don't this is the question of security but for some people isn't very convenient.
