When configuring FortiFone IP extensions on your FortiVoice system on a single LAN deployment, they will auto discover utilizing SIP PnP, in which a multicast is sent out on the network.
For FortiFone devices on networks that use a different subnet than FortiVoice, the multicast will not make it across the various subnets. In deployments using different subnets it is best to use HTTP or HTTPS with Option 66 configured on your DHCP server.
The HTTP and HTTPS protocols increase the reliability of the FortiFone devices being able to auto provision across the network. Option 66 set on the DHCP server creates an easy way to have all phones directed towards the FortiVoice in order to auto provision.
This recipe covers the best practices for a large deployment of FortiFone devices with the FortiVoice system.