VoIP/SIP/RTP issue, The incoming RTP Stream (the sound) is cut off

Similar to this thread :   https://forum.fortinet.com/tm.aspx?m=167064

Our setup :  

2xFG200E, A-P Cluster, v5.6.6

All VoIP/SIP/RTP Engines is disabled (as faar as we know how) as

config system settings

    set sip-helper disable    

    set sip-nat-trace disable    

    set default-voip-alg-mode kernel-helper-based   

end

delete the SIP session helper  (#13 in our setup)

Our Issue :

We have around 10000 calls during a normal day

On a small percentage of these calls the sound from the Caller is just gone in the middle av the call.

The RTP stream is not present.  Sometimes it´s back after 5-25 seconds.

Using Wireshark and mirror ports on our switches we can see that; * For "no reason at all" it stops forwarding RTP packages  * The incoming RTP frames are seen on incoming interface, but not on the outgoing. * When the Sequence Number rollover from 65535 to 0 in the RTP stream, it starts working OK again! We cannot see any differences with the last few UDP / RTP frames or the first frame that is not forwarded. We have created an Fortigate Ticket today.

Do u see the same issue? 

Is it possible to Disable the RTP engine inside the Fortigate even more?  

We don' t use the SIP Proxy, but could this really start working better if we use all the VoIP/SIP/RTP engines in the Fortigate? Those parts of the firewall has caused other issues back in the days.