The idea of a queue is that the agents/members of same would be connected to the phone system. The phone system manages the call and has no way to manage a cell call. It does not know if the cell phone is busy on another call and would not know to not send a call to that extension which may then answer the call with the mobile’s voice mail. You cannot transfer a call when using cellular calling. All call control is with the mobile provider and the UCM is simply a conduit by which to deliver the signal,. but can do nothing else about managing the call. Once the provider has the call, the provider has control, not the UCM. The only thing the UCM could possibly do is to ring for a shorter period of time before an answer so as to not relinquish control or use confirm when answering (which you do not have enabled). The key is that the cell phone needs to be managed such that the UCM maintains call control as it would if the extensions were SIP extensions.
A ring group could be used if you want to use the LDAP function. It has less strategies to select and queue metrics don’t matter given a ring all or ring in order being the only selections. Some of the same issues may still happen however as again the UCM has no way to monitor and know the status of a cell phone and some calls may end up in one’s cell VM.
The above is why Kevin is suggesting to use a softphone that is connected as an extension to the system via either the mobile phone’s Wi-Fi or cellular data capabilities.