SIP UDP Security issue

Hi 

I am using FGT80C with OS 5.6.11 and there is issue 

I have SIP server (210.X.X.82 ) on DMZ 

so I try to setup policy 

Policy 12 from DMZ (210.X.X.82 ) to Wan1 (GEOIP -JAPAN ) and allow port SIP (UDP5060)

Policy 14 from Wan1 (GEOIP -JAPAN )  to DMZ (210.X.X.82 ) and allow port SIP(UDP5060)

then current issue is we have attack on SIP server from all over world 

that pass the policy 12 . 

Strange thing is Policy 12 is from DMZ to WAN1 however forti cloud log shows attacker come through this policy

we try to use VPN(US IP address )  and TCP has blocked but confirmed UDP5060 can access .

is it OS bug ? or my setting is bad ?

to avoid this issue I change port to another so attach is finished 

however still this problem remain I meant if they find new port they can access . 

Other policies shows only from local to wan etc so only these 2 policy related to DMZ function