I already tried all NAT Options and always did a “fwconsole restart”.
But Problem keeps the same, with another SIP Client its working fine
OK, so let’s debug with this one. It’s obviously reassembled from (at least) two packets, because frame 248 is only 66 bytes and the SIP/SDP is much larger, though less than 1000 bytes.
What is the size of the preceding packet (more specifically, the packet containing the first fragment)? If it’s ~1514 bytes (like in your earlier post), take a look at the hex to see what junk it is filled with, which may be a clue as to what’s wrong. Also, where was this capture done? If at the far (PBX) end of the VPN, the VPN could be causing the trouble; check a capture at the PhoneSuite end.
I doubt that PhoneSuite would have itself generated fragmented packets, but possibly the OS it’s running under somehow got a small MTU value (as a result of the VPN).
Although I don’t believe it is in violation of the RFCs, sending both Authorization and Proxy-Authorization headers is very unusual and bloats the packet by ~100 bytes. I’m sure that Zoiper doesn’t do that. Possibly, the MTU value is such that Zoiper’s packet gets through unfragmented but PhoneSuite’s doesn’t.
The trace is done on the PhoneSuite side (Windows 10).
But I already tried different Windows Systems, always the same problem, with VPN or a direct connection over the Internet.
So the MTU is for ZOIPER and PhoneSuite the same.
But I will take a look at the HEX
Thank you 
The Company behind PhoneSuite gave me a new fixed version which reduced the SIP Header size.
Now everything is working fine.
Thank you all for your help
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