That is one of the "problems" when sending /forwarding any call to a mobile phone with a Voicemail option. Even if your PBX has the capability of recognizing answer supervision, if the mobile set is out of the service area, or is on another call, the PBX call can go off to the mobiles VM, and in most cases the PBX is unaware that it went to voicemail. It just knows that the call was answered.
A queue has to be able to handle calls in a bit of a different manner, as the calling party doesn't know exactly who they are reaching (in most cases) so they are not familiar as to how long it should take for the call to be answered. If the call takes too long, (the ringing part after they have been listening to music on hold for a while), then they are liable to hang up and perhaps (hopefully) try again thinking something has gone wrong. When a queue sees that an agent is idle, the call should be able to get to that agent quickly.
Because of the variables (poor signal, dead battery, already engaged), that mobile phones introduce, having one as a part of a queue, may not be the best choice.
Hopefully someone else may have a suggestion, but other than removing the VM option from the mobile, I'm not sure how else you would proceed.