What Sandbox have you reserved? Jabber guest dedicated or the UC Endpoint lab?
We have one lab that uses expressway currently. this is a shared environment and allows a user to make a jabber guest test call using the following link:
We also have jabber guest available in our UC endpoint lab. You can VPN in (once you have created an endpoint), and make a jabber test call internally. this does not have expressway. However, we are currently installing expressway servers in this lab. it will be available soon.
There is also a jabber guest dedicated environment. This builds out new CUCM and Jabber guest servers which are dedicated to you for the time of your reservation. you have full admin access to them. you can create links and make test calls over VPN. Expressway is not available with this lab.