FWIW, I think I found a workaround for this problem using Google Voice. My cell phone is forwarded back to VOIPo after several rings via GV, and the GV number is routed directly to voicemail in VOIPo. I haven't tried it that many times, but it seems to get to the VOIPo voicemail before the GV voicemail picks up, and it does not go in a loop. I also set GV to show its CID, but I'm not sure that's necessary.

By using GV, I believe you can also simul ring more than one phone, but I haven't tried it. Of course, not everyone has GV . . .

I still would like VOIPo to develop a more robust and intelligent simul ring, but I'll try this for a while to see whether it is a stable solution.