I found the following in the call centre 3.1 document.
About VDN Return Destination
The VDN Return Destination feature allows an incoming trunk call to be placed back in vector processing after all parties, except the originator, drop. This feature is activated through switch administration of the VDN form. This feature is included in the Avaya Contact Center Deluxe
package and the Avaya Contact Center Elite package.
A field on the VDN form allows the user to enter a VDN extension as a Return Destination. In this section, the VDN which has the Return Destination field administered will be called the
VDN with this feature active. The Return Destination VDN (the one specified in the new field) will
be referred to as the Return Destination.
Every incoming trunk call that is processed through a VDN with this feature active will be placed back in vector processing when all parties on the call, except the originator, drop. For this feature, the originator is the incoming party that originated the call at the time the call entered
the VDN with this feature active.