Lync 2013 Web App Sharing not working.

Question

We are having a problem with Lync Web App 2013.  Our external customers can open the meeting link and join the meeting without a problem.  Once they are in the meeting an internal user begins the presentation by sharing his screen.  The Lync
Web App just says "Loading..." for the external customer.  The internal presenter sees this external customer continually change from "in collaboration session" to "not in collaboration session".  The external customer
is never actually shown the screen share.
Other notes:
-->We tried this from an external wireless hotspot and it worked fine, thus, it seems something in this customer's company firewall that is blocking it.
-->We also tried sharing a powerpoint (we have a functional office web apps server), which produced the same results.
-->When we used the Lync 2010 Web App (before we upgraded to 2013), everything seemed to work fine, thus I am concluding that the 2013 Lync Web App (since it has more capabilities) is somehow trying to use more ports or something.
-->Internally, the Lync Web app works fine.
So, I assume something in the customer's company's firewall is blocking some access to Lync 2013 Web App.  My question is:  what needs to be enabled both on-premise and at the customer's side to access Lync 2013 Web App screen sharing and if the
company (for security reasons) doesn't want to open extra ports, is there some work around to force Lync Web App to use port 443 for screen sharing.
Thanks,
Adam

Answer 1

The ports 1024-65535 * are used for application sharing.
You can configure the port ranges for Lync clients.
http://technet.microsoft.com/en-us/library/jj204760.aspx
You can’t use 443 as port 443 is used for HTTPs.
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