Is it possible to callback
if an outside caller is busy?
Sadly no, this does not work :( Here are the Callback restrictions;
The Cisco Call Back feature allows you to receive call back notification on your Cisco IP Phone when a called party line becomes available. You can activate call back for a destination phone that is within the same Cisco CallManager cluster as your phone or on a remote PINX (PINX---Private integrated services network exchange) over QSIG trunks or QSIG-enabled intercluster trunks.
This is the same for all PBX's I have ever worked with. There is no way for the local CCM to monitor these remote phones (so it makes sense)
Hope this helps!