@Levander:
Yes, I ended up having the exact same problem, (before I had ever even stumbled across your post here, from over two years ago).
Here is what worked for me, to solve the problem:
I discovered that the GV Mobile + app would only intercept my Obi200's incoming calls, when I had the setting called "Incoming VoIP calls" set to enabled.
As soon as this setting was returned to disabled, the Obi200 device received incoming calls fine, and my GV Mobile+ app would also be able to function as much as I need it: namely, for receiving text messages, voicemail notifications, and for occasionally dialing outgoing calls... [I never really needed it or used it for incoming calls, anyway].
(I never found a need to use the GV Mobile+ app for incoming calls while at home, since I had my Obi200 device using cordless phones, instead, and this was much more convenient; but, if one desired to use the iOS app for incoming calls while away from home, then the setting could be re-enabled, but the Obi200 would be thus disabled back at home).
I hope that helps!
Please leave a reply, if this solution worked -- or did not work -- for you, ("you" being anyone who reads this, hereafter).
SPECs: I am running iOS 7.1.2 [I know I know: a little outdated for 2016...], with GV Mobile + version 3.2.6
The below screenshots show the particular settings that were described above.