PLEASE HELP!! Conditional Call Fowarding doesn't work when phone is on!

I've been trying to get Google Voice working on my OPO since I got it at the end of last November. I had it set up on my previous phone and loved it. However, on my OPO, callers hear a message saying that "this is not a working number", when they should be forwarded to my GV.

At this point, everything seems to point to an issue with the phone, not Google or my carrier (Cricket Wireless). Here's why:

Conditional Call forwarding to GV with different phone (same plan, number, SIM, except on a Lumia 520): works

This should mean that Cricket Wireless supports CCF

Conditional Call forwarding to GV from OPO, while phone is OFF: works

This should confirm that Cricket Wireless supports CCF, is configured to forward calls correctly, and that my Google Voice account is working correctly

Conditional Call forwarding to YouMail from OPO, ON or OFF: works

The phone does indeed seem capable of CCF.

Unconditional Call Forwarding (i.e. Always Forward, forward all calls immediately) to GV: works

Everything here seems to be working. Not sure if Unconditional forwarding is handled by the carrier or the phone though.

Conditional Call forwarding to GV from OPO, while phone is ON: Callers hear "this is not a working number"

This Something on the phone is misconfigured or not forwarding to the correct number?

I have tried disabling Hangouts on the phone in case there was an issue with that, as well as turning off call forwarding from my Google number to my phone, in case there was some kind of a forwarding loop going on, but still no luck.

Can anybody help? Are my above assumptions wrong? Does anybody else have this working (Google Voicemail, Cricket, and OnePlus combo)? I tried switching to YouMail for a while but the app is buggy, the message quality seems pretty terrible, and there's no automatic transcription.