I am trying to figure out a safe way to do 911 service. I would like to have all my SIP phones register with a remote Asterisk server and use it for inbound/outbound calls. However when that server is no longer available or someone dials 911, i would like the SIP phones to fail over/use a local Asterisk server which will use the PSTN to call out. Does anyone have any ideas on how to make this work. Perhaps this can be accomplished by using a SIP proxy such as SER?