Fortigate SIP translation malefunction

Hello dear friends from Fortinet,

Sorry for my tone but I have no normal words.

I spent several nights trying to understand why my Asterisk PBX can't register on SIP provider. So finally I found in captured packets the makings of a failed intellect.

1. PBX sends register packet 

src:int_IP:5060 to dst:SIP_prov_IP:5060

 Message Header

Via: SIP/2.0/UDP ext_IP:5060;branch=z9hG4bK6a3e42f7

2. Fortigate translats by NAT ........ BUT!!!!!!......it changes packet's content

src:ext_IP:5060 to dst:SIP_prov_IP:5060

Message Header

Via: SIP/2.0/UDP ext_IP:5170;branch=z9hG4bK6a3e42f7

3. Evidently SIP provider response to the fake port presented by Fortigate

src:SIP_prov_IP:5060 to dst:ext_IP:5170

Message Header

Via: SIP/2.0/UDP ext_IP:5170;branch=z9hG4bK6a3e42f7

And NOTHING MORE !!!! Because this port in not listen and even is blocked as it is not permitted by policy and therefore this packet doesn't returned to PBX. And the same thing happens with RTP packets!!!

I don't understand this half-baked intelligence. How was it tested before sale to end users?!!! Why do I have to pay and then have to be stressed reading thousands forums and manuals?!!!!! Who will pay me more than 20 loused hours of sleepless nights and my life I could passed with my family or my friends?!!!!

And I haven’t found any solution for this problem other than disabling all this intelligence.

Thank you very much. Hope you will help me briefly.