GV ringing (or in your case failing to ring) any device other than an OBi has almost* nothing to do with your OBi device(s) nor any settings on your OBi device(s), nor any settings on your OBi dashboard(s), nor any settings relevant only to your OBi device(s) in your settings at
voice.google.com.
If your GV account is working fine to make and receive calls with your OBi device, then any problem with GV calls forwarded to your iPhone is entirely between GV, your cellphone carrier and your iPhone.
(* If the OBi is somehow answering calls extremely quickly then in theory there is the possibility that GV stops ringing all end-points so quickly that none of the other end-points get it. But I really doubt that is happening.)
GV has no knowledge of, and frankly doesn't care about, what OS version or even what make of phone it is forwarding calls to.