I've had reports that the caller hears two rings before the OBi Rings.
However, back when I was using a Linksys ATA, it seemed to be the same way.
In fact, I can often call a friend's Cell, from my Cell, while standing next to him. My phone rings twice before his rings once. We both use Verizon.
I don't know how relevant that is to you, but it seems the real consideration is; how many rings the OBi ring before going to VM.
-G