Hmm, yeah I suppose that would be another way to achieve this: appropriately chosen max wait time, agent timeout (with max wait time mode maybe 'loose' ), retry value that fits several times in the max wait time, auto-pause on busy + join-empty = no, leave-empty=ultra-strict;
Also maybe the route to the mobile phone, put on as FMFM to one of the extensions, not as direct destination of the queue? Just a thought.
But IANAE 
etc, etc.