Hi Tux John,
The behavior you need is cover in Asterisk within a Queue.
1. Create a new queue in queues.conf and assign as static member, the 4450
2. In your dialplan, you need to route the incomming calls to the new queue and pass as argument the timeout in seconds you want when hangup the waiting calls.
When a new call arrives, it enter the queue. The Callee ear moh music, while the 4450 ring if its available. Ir not, the queue system wait until the 4450 become available, an then send the call.
Please, refer to http://www.asteriskguru.com/tutorials/queues.html
The #3 title (simple queue) is all you need.