Hmm I wonder if this could be one of those very rare Android phone or Vendor OS that does not allow the native dialer from other apps (which should be a no no but some of them do it).
When you installed the app, did the phone take you to a screen called calling accounts where you had to turn on a switch for Vodia Phone 2?
If it's available you should still be able to find it in the native dialer. In the native dialer, not the phone app, under setting then "other call settings" or "calls" (it's different with diff vendors), then Calling accounts, then active calling accounts, you should see Vodia Phone 2, turn it on to give Vodia app the permission to use the native app. But as I said this should have come up on initial install if your phone supported it, or on incoming call.
You can also open a ticket so we could share info for a video call/screen share if you want.
Thanks.