I have an unusual issue with Call Park feature not working on incoming calls from my ITSP.

I have configured my Aastra Handsets with a Call Park Button

Using:
sprecode: *5#
Label: Park
Line Selection: {disabled}

This works perfectly for internal calls between handsets (PBX - PBX)

However this fails miserably when dealing with external ITSP calls.
The handset either responds with a Failure message or a Busy Message

I have also experienced an unusual behaviour when parking an ITSP call manually.. where the system does actually park the call but does NOT provide any voice announcements as to what extension the call was parked on...

Ideally I would like to replicate the success of the call park button on the PBX - PBX based calls.

Any ideas would be greatly appreciated.