There is, but you need to enable Mobility for Single Number reach. Basically, when someone calls their deskphone, it will ring the remote destination phone such as a cell phone at the same time. If the person does not answer at their desk OR their remote destination (cell phone) it will pull back and go to voicemail off CUCM.
It's built into CUCM 6.x and 7.x. Works good, BUT, it takes up more PSTN services because of the inbound call then outbound to remote destination.
Users can program their own numbers in through the CCMuser page.
hope this helps.