VC issue

We are facing strange problem to using video conference. My current setup Fortiwan-------->Fortigate----------->Polycom 500 Group real presence. My problem is if we are calling from Public IP(Natted Device) to our VC it's work perfectly but if I called from private ip over the internet i.e. Mobile to VC device.It's display black screen and audio delay issue.

Noted down with enable h323 alg enable device is working fine.

Is any clue where is suspect.