Will you have any kind of 'alternate phone number' option (failover)? If my server is down and a call comes in, the call hangs for a while (dead air), with an eventual recording that says the call did not go through. In this kind of situation, I'd love to have an automatic failover routing to my cell phone...

For what it's worth, if an invalid SIP server is specified, any callers will hear a 'number disconnected' intercept message.