Let's see if I have this straight, "A call comes in, they dial your extension number, on a busy or no answer the caller goes to your mailbox. You want to give them the option to leave a message or transfer to your cell...."

Try this...set up a single line extension to forward to a system speed dial bin number by in the G screen for that single line set all calls CO to Yes to *099 in the day and night g screens.
at the operator's phone program bin number 099. [pgm] *099 *4 XXX-XXXX [pgm].
in the mailbox set operator to that single extension or use ESP to establish a blind transfer to the single line.....

Hope this helped!

It is better to have and not need, than to need and not have!