I think I have resolved the issue here... and as you probably suspect, it has nothing to do with my Obi devices or google voice. Apparently, deep within the settings of my cell phone are options for call forwarding for when my phone is busy, unanswered, or unreachable. The settings for busy or unanswered go to voicemail, the settings for unreachable forward to the number that is now being used for the google voice number on my Obi device #1.
So calls to my 2nd google voice number are forwarded to Obi #2, and to my cell phone... which, if it is unreachable, forwards to the number that is now associated with Obi #1.
My best guess is that my phone has always been configured this way out of some courtesy by my cell phone provider, since that number is listed as my "home" number in my cell provider account. I don't think I ever set that up this way because I don't remember ever seeing those settings before, and because, for privacy and security, I'd have never forwarded calls to my cell phone to THAT number.
It's only the past few months I've noticed it because it's only been the past few months I've lived where I get little to no cell reception... making my cell phone "unreachable" a lot more often.
If this for some reason doesn't resolve my issue... I'll make sure to post and let you know!
-Nostradominos