I have vlan.0 (192.168.2.0/24) and vlan.20 (192.168.20.1). Polycom VOIP phones boot up in vlan.0, then inspect dhcp option 128 to get their vlan id (20). The phone then drops the vlan.0 ip address and re-boots with DHCP server address 192.168.20.1.
The SRX dhcp server at 192.168.20.1 can be pinged from the 192.168.2.0/24 subnet, but the phones are unable to get an IP address in the 192.168.20.0/24 subnet. I think the problem might be the propagate-settings interface, but I'm too new at this to know for sure.
# show | match 192.168.20. | display set
set system services dhcp router 192.168.20.1
set system services dhcp pool 192.168.20.0/24 address-range low 192.168.20.2
set system services dhcp pool 192.168.20.0/24 address-range high 192.168.20.254
set system services dhcp pool 192.168.20.0/24 router 192.168.20.1
set system services dhcp pool 192.168.20.0/24 propagate-settings vlan.20
set interfaces vlan unit 20 family inet address 192.168.20.1/24
#voipdhcpsrx