Then maybe I should try the simulring test that I was asking about recently: I would do the same thing, except set my cell phone for no-answer transfer back to my VOIPo line, so that the call will go to VOIPo voicemail instead of my cell phone voicemail.

In your third test, it looks like VOIPo recognizes the cell phone number as a simulring number, and therefore does not simulring to it. In my case, the CID would probably show the original calling number rather than the cell phone number, even though the cell phone is forwarding the call. In that case, VOIPo will probably NOT recognize the number as a simulring number, and will therefore ring the cell phone, which will forward it back to VOIPo, which will ring the cell phone, etc.

I don't know how difficult it is to get VOIPo to recognize that the forwarding phone is a simulring number, but it would be a very convenient feature for me. I prefer having all of my voicemail in one place.