I know this is months from where you were, but assuming that the elevator is the one sending the DTMF, you could make a Misc Application that goes to the Terminate Call application, Assign it to some arbitrary extension (i.e. 900), and then have the elevator phone send ## {pause} 900#.
The ## will initiate a transfer, pause to give a second for dial tone, and then 900 being the extension, # to complete the transfer, transferring to a extension that hangs up on you.