A while ago my, Skype for Business Client on my Mac was unable to connect while I was in the Office. So I switched to our Guest Wifi and did some check (because it’s always the Firewall, and the guest network is nearly open – That is an insider, sorry!) and it just worked fine.
After joining our regular network again, the Client stopped working, on WiFi and Ethernet. The Firewall logs didn’t show anything useful. The Skype Logs itself pointed me to the right direction: Connection issues!
So I started to dig deeper and checked all the hosts Skype for Business tries to reach from the command line (Curl is your best friend in such cases). Tools like Wireshark could do the same job.