Do you have any gateways? if so, you might need policies between that and the phones.
Can you explain more of you voice topo? Example, do you have designated voice only vlans?
If you have deisgnated voice subnets, you fwpolicies can be simplified
e.g
allow voice subnet 10.1.0.0/24 to voice subnet 10.2.0.0/24
allow TCP SCCP 2100/tcp
allow MGCP 2427/tcp ( probably used for gateways )
Don' t forget the need for DHCP and TFTP-services
So basically the phone neesds access to the call manager and then to each other. the RTP streams should be in a udp port range 24XXX to 24XXXX or something along that line.
I' m including a cisco link to all of the ports within cisco jacked up voip deployment ;)
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_tech_note09186a00801a62b9.shtml
If I recalled correctly, i thought FGT supported a SCCP aware ALG, but I could be wrong.