You should be able to test the Jabber Guest shared sandbox URL in any (supported) browser, using the URL:
https://jabberguestsandbox.cisco.com/call/5555
If this works, you should be able to use the same URL in testing apps.
Note the Jabber Guest server at jabberguestsandbox.cisco.com is shared, and so we have not enabled access to the admin link API. You can reserve a dedicated JG lab via the DevNet Sandbox, which should provide you with the admin credentials.
For best support with Sandbox related issues, please see the Sandbox forum here:
https://developer.cisco.com/site/devnet/sandbox/faqs