If you have MAS (Mitel Application Server) 4.0, you most likely have a Mitel MCD 3300 system using SX-2000 Light peripheral bays for the digital sets (the set type itself is actually not relevant). You will need to setup an EHD user, using External Hot Desking, and to get the "pull back" of the voicemail you need a PRI or SIP trunking, setting up the user as an EHD user should give you almost exactly the functionality you want except the desk phone and cell phone will ring at the same time rather than being forwarded but this can be toggled off and on. You will need an available EHD license to do this though.
Otherwise, through standard forwarding I don't think you can have the call forward to a cell phone and then come back to the Nupoint.
One other option might be to setup an FCOS in the Nupoint that allows for cell phone dial-thru and forward the phone directly to Voicemail, and in the user's greeting they say something like "Hi, you have reached Steve with XYZ company, to reach my cell phone press 2, or stay on the line to leave me a message" and when they press 2 they are placed on consultation hold and the cell phone is rang but if it isn't answered the call reverts to the Nupoint mailbox. I can't tell you the exact steps to set this up off the top of my head, and it's been years since I've done it, but I am sure if this sounds like it will work there are several people here who could probably walk you right through it.
(I am going to leave this topic here for the moment, but depending on the next post or two it may get moved to the 3300 or Application sections if they are more relevant)
