DHCP Server Used for Failover

We have a SIP application where in we would like a SIP device to resolve an FQDN and be sent to the public (hosted) server while the internet link is up. However in the situation where the internet goes down we would like Fortinet to resolve the DNS to a local IP address (backup server).

I looked at the Fortinet DHCP server and as it I don't believe it will work because Fortinet will resolve the request using its internal table first, and then only if a match is not found will it pass that request up to a DHCP server on the internet. This will not work for us because we want the SIP device to register to the remote system at all times and only register locally should the internet connection become unavailable.

Does any one know how you would get Fortinet to pass all DHCP requests to the internet DHCP server when possible and only resolve based on the static DHCP tables in Fortinet when the internet connection becomes unavailable?

Note: I know that many SIP devices have a primary and fail-over IP or FQDN fields. The SIP device we are using however does not.

Any suggestions or follow up questions welcome. Thanks in advance for help with this.