Rodger,
What probably happened with your ios user is that she updated to the latest version of the ios client which came out in the past few days . That now supports call queues. In the older version call queues didn't work with mobile clients. So looks like you have users on two different versions of the client. As others upgrade the ios client you will have the same problem with them.
You will probably have to reconfigure the way your call queues works (which should be relatively easy) so that you have different call queues for different shifts and the Auto attendant routes to those different call queues if you want the user to be able to have her SfB device still logged on but not recieve calls originating from call queues. So, your user might be in call queue X (early shift ) but not in call queue X (late) and the Auto attendant will switch betwen which call queue it routes to automatically.
I agree it would be a good feaature to have an ability at the client end to subscribe or unsubscribe from different call queues rather than have to log off the client in order to remove themselves from the queue.
I hope that is helpful
Regards,
Nicholas