Create a ring group. On the extension list for the ring group, put:
101
2135551212#
Then in the inbound route, select the ring group as an extension. It will ring both your internal extension 101 and your cell phone number at 2135551212 at the same time (you can send it to any outside number by just putting # at the end). Whichever one you pick up first gets the call. Of course, you can build onto this if you have other needs. Hint: If you want all unanswered calls to go to your cell VM, set the ring time long in the ring group. If you want unanswered calls to go to your PBX, set the ring time short. Experiment to see the correct ring time for you.