I would probably set this up to work like this:
Create a Call Routing Application that has no greetings and times out immediately to the VM box of the overflow phone.
Create a new hunt group and add only the overflow phone as a member.
Set the recall timer to whatever you want the delay to be before it goes to VM.
Enter the number of the Call Routing Application you have just created under the Recall destination.
Create a new System Forwarding path that routes to the new Hunt Group.
Assign this System Forwarding Path to the phone that rings initially.
So it should go Call rings in to initial phone and is unanswered > forwarding timer kicks in and routes to hunt group > Overflow phone rings until recall timer kicks in > Recalls to CRA that routes straight to the overflow phones mailbox, caller leaves a message and everyone is happy.
In theory