I've found that on both the iPhone and Android Jabber client, if you sign out, you can't immediately sign back in and register the device. Is there some sort of time delay for CallManager to accept another sign on? I find myself having to reset the device from CallManager as it shows registered, but it's not as far as the phone is concerned.
The client shows signed in (the sign out button is visible) but the phone services are not available.
Any thoughts?