Minimum ports and protocols requirements used by Skype for Business/Lync clients:

Port Protocol Notes
 5061  TCP (SIP over TLS) Main port used by Skype for Business/Lync clients
 443  TCP (HTTPS) Used by Skype for Business/Lync client for Audio/Video Conferencing
 444  TCP (HTTPS) Used for data sharing and transfer during meetings

If your Skype for Business/Lync client cannot connect to the server or you are unable to initiate an Audio/Video call, start a meeting or use any other supported functionality, verify that corresponding ports are open on your firewall by running Telnet to the Skype for Business/Lync server.

Syntax:

telnet [RemoteServer] [Port]

[RemoteServer]: specify the name of the server to which you want to connect. Skype for Business server name can be found under HostPilotĀ® Control Panel > Services > Skype For Business > Instructions > Server Name

[Port]: enter the port that you want to test.

To ensure the best possible experience we recommend to also allow connectivity to our Skype Edge servers to the following ports. 

Port Protocol
3478  UDP
 50000-59999  TCP 
50000-59999  UDP

For more detailed information, contact Support.