VVX 400 park calls

I have a question regarding park calls using a Polycom VVX 400.  

Senario: 

1. Incoming calls gets answered and placed in Park 1
2. another calls comes in and gets placed in Park 1

Now I have two calls parked in area 1.  And the two callers are dragged into a conference.  

I tried doing the same thing using a VVX 500 phone and it does not allow me to park multiple calls in one park area.  How do I implement the same feature with other polycom phones  400/300/301

Thank you