You probably have the something associated wrong. So here is the call flow:
Someone calls your desk line on the IP Phone. (line associated to username)
On your Remote Destination Profile, you should have your desk line also configured as the line and associated to your username in CUCM.
Your end point is your cell phone. You may have to play with the triggers on your cell phone profile for when it should ring.
Also, turn the on more rings at your desk. The cell phone network needs at least 2 rings just to get your cell phone. So one ring your desk, second ring your desk, out to the PSTN, 3rd ring, your cell phone/desk 4th ring cell phone/desk 5th ring cell phone/desk 5th or 6th ring desk or unity vm.
The mobility softkey simple turns the end point cell phone on/off.
