Hello MichaelN,
welcome to the Polycom Community.
This completely depends on your SIP Switch.
Parking a call sends an INVITE to a park position. In order for the call to come back the SIP switch needs to send us an INVITE.
Not knowing what type of SIP Switch you use please work with whoever installed the system and the phones.
Please ensure to provide some feedback if this reply has helped you so other users can profit from your experience.
Best Regards
Steffen Baier
Polycom Global Services