This would be referred to as three-way calling or call transfer, being provided by the dial tone provider. To use this feature, the system users will simply use the flash key (the button with the lightning bolt on it), receive a second dial tone and make the second call. They can then hit flash again to create a conference. If the lines are configured for call transfer, then they just hang up, leaving the distant parties connected. This really isn't a function of the phone system except that you'll need to make sure that the CO flash timer is set for 600 milliseconds.

Ed Vaughn, MBSWWYPBX