[Problem] Stange issue with RTP NAT

[stefanoIT]

I'm using a 7362 SL as DSL router.
I have a SNOM 710 in my LAN (192.168.4.0/) and I'm having problem with RTP NAT on FB.

SNOM is registered to a OpenSIPS proxy with public IP.
I did not configure STUN in the SNOM because OpenSIPS fix the SDP header with correct IP.

it happens that when I place a call I hear no audio.
After many investigations I found that Fritz Box doesn't open the Firewall

The followind UDP packet has no effect on FB, the firewall remains closed.

the UDP packet sent by SNOM has no effect on FB

44	7.200875	192.168.4.53	peerIP	CLASSIC-STUN	70	Message: Binding Request  
Frame 25: 70 bytes on wire (560 bits), 70 bytes captured (560 bits)
Ethernet II, Src: SnomTech_76:e5:dd (00:04:13:76:e5:dd), Dst: AvmGmbh_2a:bb:c1 (08:96:d7:2a:bb:c1)
Internet Protocol Version 4, Src: 192.168.4.53 (192.168.4.53), Dst: peerIP (peerIP5)
User Datagram Protocol, Src Port: 52476 (52476), Dst Port: 7078 (7078)
Simple Traversal of UDP Through NAT
    Message Type: Binding Request (0x0001)
    Message Length: 0x0008
    Message Transaction ID: 14735ac55e4b79633b706424119e09dc
    Attributes
        Attribute: CHANGE-REQUEST

I did another test with an asterisk server in my LAN. The result was the same.
If the audio peer IP is different from signalling proxy IP the firewall are not opened in the FB.

I discovered that FB trace function do not trace RTP packets so I cannot see what happens on FB interfaces.