There are devices made for home use with cell telephones that will allow you to pick up a s/l phone and make and recieve calls on the cell. I would hazard a guess that, if something like this were wired before the system and into a line port, then, theoretically, anyone dialing out on that particular line would dial out on the cell. In the versions that I have seen, the cell phone must sit in the adapter base for this to function.
I have never heard of anyone doing this, or wanting to do this on a phone system, but I do not see why it would not function.