What phone app works if you are using Quick books V15.0.5R6

While using Android, I've downloaded the "QuickBooks Online" app and it is working fine. All the best :)

iOS app To download QuickBooks Mobile App: Open the App Store on your device and search for QuickBooks Online.(It may not be available for some countries.) Select Free, then Install. If you are already signed up for QuickBooks Online, use the same user ID and password to sign into the app. You may be asked to confirm a one-time confirmation code to confirm your identity. Once confirmed, you can continue into your account. Android app To download QuickBooks Mobile App: Open Google Play on your device and search for QuickBooks Online. This app works on smartphones and tablets. (It may not be available in some countries). Select Install.

QuickBooks Accounting: Invoicing & Expenses in Google Play store. I'm sure there is an Apple version.

I would download the app and try to log in. You'll know if you're able to login using your desktop login credentials. It syncs across devices so you may have to have QB Online in order for it to do so.

All you need to do is download the app and install. It will offer you all you need,while you operate it from your mobile. It's very easy to use.

You are correct, you can only use the Invoicing app if you have the online version of Quickbooks. Hope this helps!