Yes, I do have it enabled. And, like I said, it works fine (I do get the stutter tone/flash when I am on an active inbound call and I get another inbound call), because Callcentric's systems serve both calls. But when I am on a outbound call and I get an inbound call, it goes straight to voicemail.
Here is what Callcentric customer support had to say "We haven't tested any other providers, but this should be a limitation with all VoIP providers. Everything in SIP/VoIP is IP based which is packet based it is not voltage based like with PSTN lines. We do not receive a packet telling us that your phone is off the hook. There is no way for us to detect on hook state because devices do not tell us whether they are on or off hook. With old PSTN lines the voltage came from the phone company so they always knew the state, however that is not the case with SIP"
So that's that I guess :-(