I'm sure you've seen this doc, but it is worth checking out if you have not. http://www.cisco.com/c/en/us/support/docs/unified-communications/jabber-guest/118592-technote-jabber-00.html
Next, have you reviewed your chip.log on the test PC?
This smells like a possible TURN problem and the chip.log will be really helpful.
Hi Mark. Thanks for the reply. I have seen that doc. The issue turned out to be how the "Default caller SIP alias" is entered under the Links menu. Poor user interface here on Cisco's part. When I read "sip alias", I think user@hostname. But Jabber Guest is only looking for the user portion. Entering the full alias caused all calls to fail. In my opinion they should re-word the title of the field and provide some sort of validation to throw an error if the full uri is entered.
Ah yes, been bit by that one too. Glad to hear you found it!
If you have access, check out the June 24th 2015 Jabber Guest presentation on PEC. It was very helpful for me.