You can setup a DID that users call into, it authenticates based on their cell phone number (needs to be entered under a user's mobility tab), and gives them system dial tone to dial out from. It uses 2 channels because one for the user to call into from and one for the user to call out from. When I worked remotely I used this method to make outbound calls and inbound were just twinned to my cell. It worked fairly decently although we parked calls and I had to user shortcodes to pickup and park calls that part was kind of a pain.
The truth is just an excuse for lack of imagination.