Send the call out of the VPS and to an ICD group. Put a message card into the system and set up the group to play a message when all members are busy, or alternatively after a certain amount of time.
If you have to do it through the VPS, try to use a TVM. It isn't the ideal solution, but you could have the group overflow to VM on busy, and setup the personal custom service for that mailbox (call will take follow on ID and route call to the ICD group's mailbox which will be the same number as the group's floating number). You then can tell the caller to dial one to be transferred back to the group (transfer call to group extension) or hold to leave a message. Then either auto forward the message to a mailbox that has a phone, or create a message waiting key on the 2 C/S phone for the group's extension and both will get indication of message being left...