A major difference between a PBX and a traditional telephone is the concept of Trunk Lines and Call Presence.  Your phone may have more than one "Call" button which indicates a position for a concurrent call between your phone and another party.  This is separate from the actual Trunk Line (connection between the PBX and the outside caller) that the call takes up.  

This concept allows you many features including the ability to have more calls on your system than the limitation of physical feature buttons on your phone.  Of course, this leads to some differences in function.  Most obvious it the use of Hold vs. Park.

Hold

Activating the Hold function from either a Hold soft-key or a physical Hold button will cause the currently active call (indicated by an illuminated Call Presence button) to be put in hold with music.  When using the Hold function, the call is held on the phone where the call was initiated.  Hold is a private function.  This call remains on this phone and cannot be picked up from another extension.  A common use case is to put a caller on hold while you call another person or go to check on something.

Park

The Park function is similar to hold, but frees up the occupied Call Presence button and can be retrieved from any system phone.  It is possible to limit which phones can retrieve a Parked call, but that configuration is beyond the scope of this article.  When a call is placed on Park, the caller is put on hold with music (similar to the Hold function) and the call is available for pickup from the same or another phone.  

If your phone has "Park" feature buttons, you may store a call in that corresponding park slot by simply pushing the button.  The call may be retrieved by pushing the corresponding "Park" feature button on the same or a different phone.

If your phone does not have a "Park" feature button, a call may be placed on Park by dialing *** (asterisk three times).  After dialing ***, the caller will be put in the first available park slot and the system will read back the park slot number to you (ie: 701).  While in Park, the caller will hear hold music while the call waits to be retrieved.  To pickup the call from a system phone, dial the park slot number (ie: 701).  You will be connected to the Parked call.