I am not a Firewall person. Lets just get that clear. I want to make sure I don’t forget this. I did run into this with my mentor and a customer who had an inbound outbound Lync issue for external access. I turned out to be the blood hound in this situation. The Fox I had cornered was one that does not show up well on a packet trace. It appears like a tarpit. Packets response times increase in latency. There is no exact error to speak of. If you look at a trace you may find this under
So you will add your policy rule for Source NAT translation for your AV firewall rule. Next you will disable Palo alto syn cookies. This setting may cause latency with lync communications.
I included an example Wireshark TCP graph of what the failed transmits may look like. If this graph shows a straight 45 degree angle line, then this is likely not your issue.
This is only one of several settings that may cause this. ALG filtering is also the same graph. I Just happened to find this on Palo Alto but any Next Generation Firewall is likely to show this issue if the source port translation, destination translation, and Firewall filtering settings are not corrected. I hope this is helpful and Lync On.