Create a phantom extension. Manually fwd that ext to the outside number. Put a button for that ext on the phones.
Or...
Set up Dynamic Extension on the phantom so when the phantom rings (virtually) so does the external number. Added advantages of this are that phantom can have voice mail and if user is ever in the office he cal hot desk into an office phone.
To manually fwd a phone: from a working phone (display with soft keys is best), dial 359 (REMOTE PROGRAMMING); dial phantom; dial phantom pass (by default same as ext) followed by #; dial 355 (FWD ALL CALLS); press OUTGOING or 8; dial outside number; hang up. Be sure OUTGOING on phantom is correctly configured for ARS or trunk group being used.