To anyone facing the same question in the future I thought I'd post up the word back on this from Inin (a colleague with appropriate access logged a case on my behalf), which makes sense to me.
Abandons:-
Abandoned queue interactions occur on a distribution queue when a queue interaction disconnects before it enters a Client_Connected state (that is, an agent or user picks it up).
These example scenarios can be considered abandons:
o Disconnects before the queue interaction is connected to a user or agent. This only includes remote disconnects.
o Interactions that go to a voicemail box assigned to the ACD workgroup. To the ACD system, they are simply seen as queue interactions that disconnect while on the workgroup queue.
o Interactions that go to a voicemail box assigned to the user/agent queue where the agent never answers the interaction.
Service levels:-
Service level statistics in the Interval Queue Data tables are based on the number of interactions answered within the specified number of seconds by the queue.
Thus, call that go to a voicemail is consider as abandoned call and will not be included in service level statistics.
While the callback interaction counted the same as normal call interaction.
Whenever a user disconnect from the interaction and decided to place a callback request. It will create a new interaction ID and queued into workgroup.
Callback scenario resulted in a total of 2 Interaction received and 1 interaction flow out.
Below is a sample scenario how the interaction being counted:
1 user call into the queue (Interaction received : 1)
2 user decided to disconnect and place a callback (Interaction received : 2, Interaction flow out : 1)
* The flow out is come from the 1st interaction
3 agent pick up the callback (Interaction received : 2, Interaction flow out : 1, Interaction Answered)
So in summary callbacks might play around a bit with other views (where interactions received and flowed out are factors) but for SL distros the 'answer' is used once only when it is actually connected, caller to agent, as you would expect. Voice mails to me are NOT answered but are either dropping out to vmail on a pre-determined timeout or deliberately sent there (because they cannot be handled) so they should be treated as such.