"I dont know about USA version,but the Irish version of the kxtda is now solid on forward and is changeable in the systems options 1"

OBT this is a TA824. I don't recall ever seeing the option to change it on this KSU.

"If the call is already being handled by the VPS it works as expected - Busy/No Answer transfers back to VPS with option to hold/leave message."

Yeah, your VPS is doing a supervised transfer. There is no way around needing to forward your phones to voicemail if you want manually transferred calls to go to the mailbox. On the other hand, here's an opportunity to upgrade the client to a TDA. There you can change the option for a solid fwd/dnd light, as OBT mentioned above.