In the "Lab Details" document under the Jabber Guest Sandbox Lab, the following is listed:
This Lab does not Contain Expressway. VPN is required to make calls to the sandbox. If you want to make a Jabber Guest Call to a Static endpoint over Expressway
It seems as though there's something missing from that paragraph. I'm curious if it's possible to make a Jabber Guest Call to a static endpoint over expressway either from this lab or a different one.