1.Pls port forward on your router if your ippbx is back of router
RTP port range 10000-10500 (UDP)
SIP port 5060(UDP&TCP) our coocall of iphone is with tcp transport
2.Config the relation NAT
Advanced---->Options--->SIP Settings---->NAT Support
Local Network Address 192.168.1.0/255.255.255.0
External IP:it is your router wan port ip,that through to internet
Local Network Address :it is your IPPBX IP segment
3.Create one extension,then enable NAT in the extension
4.Then you can register and call each other.
also you can refer the attachment document.