PhonerLite sends requests to SIP server with incorrect data in "Contact:" field. It contains local IP (within LAN) and not the IP, that should be used to contact to it from outside the LAN.

Example:
Wrong way - "Contact: <sip:504@192.168.1.200:5060>" 
Right way - "Contact: <sip:504@XXX.XXX.X.XXX:5060>"
Where "XXX.XXX.X.XXX" is an external IP (WAN address of the NAT router).

As a result, it didn't receive replies, sent back by server, using that local IP.

When it doesn't work?
SIP server is within LAN, let say 192.168.1.0/24, WAN IP: AA.AA.AA.AA
SIP client is within LAN, the same 192.168.1.0/24, WAN IP: BB.BB.BB.BB
In this case, e.g., SIP server will try to send replies to wrong computer within its own LAN...

Yes, indeed. Via header generated by PhonerLite contains the "rport" parameter. But it's always empty. Here is typical Via string:

Via: SIP/2.0/UDP 192.168.1.10:5060;branch=z9hG4bK005fb612c1b6e011b13ce1dd6943abfb;rport 

Please note that this is the whole sting, where it ends exactly with word "rport". There is no port value and there is no "received" parameter, which could contain WAN IP. PhonerLite sends all requests (REGISTER, SUBSCRIBE and INVITE) that way (with "rport" empty).

I'll try UPnP or STUN later and report back.

Setting in configuration "UPnP NAT" changed a lot of things. As a result, in the REGISTER request local IP was replaced with WAN IP (X.X.X.X) in different places:

Via: SIP/2.0/UDP X.X.X.X:5135;branch=z9hG4bK005867f4eeb6e0118006002185e6cfdf;rport
Call-ID: 007C6432-EFB6-E011-A227-002185E6CFDF@X.X.X.X
Contact: 6FD91>" 

And now SIP server can send INVITE request, using proper IP (provided WAN address).

Thanks a lot!

While it's not closely related to the topic, and because I don't want to open a new thread just to make these unimportant points, let me ask you to make some very small improvements in GUI:

• In "Debug" window, there is context menu, that offers "Delete" (or "Cut") menu item(s), which are (at least it seems to me) always grayed. Could you please make "Delete" work. It's be helpful to watch just the recent events, related to the test, without going through sometimes a very long log (in small window) and/or restarting the PhonerLite (just to clear the log). It'd be nice to make it: 1."Delete"; 2.Test; 3.Check debug; 4. Go to p.1 again... 
• Debug tab disappeared, when I uncheck "Options | Debug" menu item. But when I check it again, there is no "Debug" tab... I spend a lot of my time, until I discover that, when I put check at that menu item, I have to go additionally to "Help" menu and click on "Debug" there too. It's not intuitive at all...
• "Logbook" tab keeps resetting width of its columns. For example, the "Date" column is almost x2 times wider then I need.]

I hope to see these improvements here someday

Thanks for the nice tip with DEL key. I promise, I'll use it quite often since now

I may see the difference, but I don't see the point of generating debug information and not showing that If I want to debug, I usually want to see that debug information too

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