Queues not skipping busy agents

Having an interesting issue with Queues:
We have tried setting a queue to skip busy agents ‘yes’ ‘yes+(ringinuse=no)’, and ‘queue calls only (ringinuse=no)’; none of these prevented a queue call from ringing an extension that was still on a previous queue call on a cellular device or softphone that was reached through a follow me.
It seems like ‘yes+ringinuse=no’ was designed with this functionality in mind but our tests have shown us that receiving a queue call on a cell and staying on that queue call does not prevent the system from sending you another one.
An interesting note is in iSymphony we can see that a queue call is being taken on a cell (provided the user has the cell information updated in their UCP), but the PBX does not seem to recognize that same call as a queue call to prevent more queue calls from reaching that user/agent.
Any thoughts or advice welcome.
also opened a bug report: http://issues.freepbx.org/browse/FREEPBX-12778