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