"Is there any way to have Unity monitor the calls to his cell?" Yes, if the PTSN connectivity off of the GW supports a discrete call progress state like PRI (if it's analog FXO, forget it). What this will do for you is release the transfer upon notification of the cell phone user answering their cell phone, or "pulling the call back" if the do not answer their cell phone.
"Upon receiving a call on his cell, he would like to be able to transfer the call back to another Unity Subscriber if the caller needs to be transferred to someone else. " That really doesn't have anything to do with Unity. Can he/she transfer a caller (to the cell phone) to some one that lives down the street? Probably not. The cell phone transfer support is going to depend upon the wireless service provider (even if the call was originally transferred to the cell phone from Unity), not Unity.
Steve Olivier
Software Engineer
Cisco Systems
