Answering my own question, I set up a cell phone number for simul ring, then turned the cell phone off. When I called in (from a different cell phone), the call did NOT go to voicemail; it rang my house phones. That's the good news. However, I did get some odd behavior. One time the called phone kept ringing long after the calling phone had connected to voicemail. Since the two phones were close together, I ended up recording the ringing phone on my voicemail. Another time, I tried to answer the phone, but I couldn't connect, and what I think was the cloned line kept ringing and ringing. I could stop the ringing only by "answering" all the extensions in the house (they didn't connect), and then waiting about five minutes before "ending" the call.

Some of this may be due to quirks in my two-line phone system. But if you use simul ring with a cell phone, I would suggest leaving the cell phone on.

I should also mention that I tried setting up simul ring with my Google Voice number, which in turn was configured to ring two cell phones. This actually worked reasonably well, even with one of the cell phones turned off. However, since GV does not let you adjust the number of rings before a call goes to GV voicemail, I was not able to force an unanswered call to go to VOIPo voicemail except by shortening the number of rings on the VOIPo side, which caused VOIPo voicemail to answer sooner than I wanted it to. I might be able to fix this by enabling call presentation or screening in GV, which would require pressing a key to accept the call (similar to "Confirm Call" suggested by fisamo), but I haven't tried it yet. My wife is getting annoyed by all the ringing.