It is working as designed.
Google Voice has a "Voicemail access" option, which determines the behavior when calling your own Google Voice number from one of your "linked" or "forwarding" telephone numbers.
The default behavior for phones classified as type = mobile, is to not ring the other forwarding destinations when calling into the GV number. This is so that people can easily check their voicemail by calling in.
The default behavior for phones classified as land lines (type = home or type = work) is to first ring all the other forwarding destinations.
You can change this in Google Voice settings. In your case, since you do want to call your GV number from your cell phone, you'd set "Voicemail access" to off.
This setting isn't easy to find, because the newly-redesigned GV web user interface left out a lot of settings.
From a laptop/desktop web browser, you need to go to your Google Voice account and click the three-bar, "hamburger" icon in the upper left corner, then scroll down to get to "Legacy" GV. From there, you can click the gear icon in the upper right corner to get to Settings.
- Click the Edit button under your desired forwarding phone number.
- Click "Show advanced settings.
- Change Voicemail access to "no".
- Scroll down and click "Save"
Do
not simply delete your cell phone off of the list of forwarding phone numbers, as this will remove the ability to easily check your voicemail from a phone other than the OBi-attached phone.