DHCP option for IP Phones

dear all,

  i have one problem 

I need some help regarding DHCP  for IP Phone ,

Yesterday we deployment Fortinet 100D .

They have 3 network and using Cisco SG-300 52 port manageable switches

1-    IP Cameras-192.168.15.0/24

2-    IP Phones-192.168.16.0/24(Grand Stream IP Phones)

3-    PCs and server -192.168.22.0/24

They want to use port of Fortigate as switch also ,

so I configure 4-4 port for each network ,

with no DHCP for IP Cameras & IP Phones network.

PC network using DHCP but they want IP Phone also to be DHCP .

This my problem area , how to configure  2 DHCP network and tell IP Phone  to take Specific IP Network 192.168.16.0/24 only .

There is any specific setup in Fortigate to this or I have to do from cisco switch.