I am sure many users here have signed up for accounts, but through a mobile app.
If you click
Are you an ObiHai User? Click here first! at »
ring.to/setup/ they still say they support only Obi devices purchased after May 1st 2014.
As far as I understand, they don't plan to support third party ATAs. Setting up an Obi without using the portal means you will need your SIP credentials, so technically they could be used on any ATA. So that would qualify as a third party ATA.