what topology have you reserved and what is your sandbox username?
I reserved a Collaboration 10.5 sandbox / username aaron-counterpath.
I re-tested again today but observed the same behaviour. Should the iOS Jabber client be able to log in using the out-of-the-box sandbox config and the pre-provisioned users? e.g. email@example.com should be able to log in and call/IM to firstname.lastname@example.org