the All-callback option might already provide what you are looking for.
With all-callback you have to specify which tariff to use to make the callback. therefore you will be able to specify a rate and a trunk for the call ... hence you could define a specific carrier and rates for your callback service
Unfortunately you will not be able to use the callerid to authorize
as to billing and how it is shown: both legs are billed and shown separately they are not combined but if memory serves me right there is a tag in the CDR that clearly shows it as a "callback".
IMHO this is the prefered way to do it because when you try to match up [reconcile] calls you want to see and show all the calls you don't wnat to have any combination. just my opinion so don't get bent out of shape
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BTW a2billing can handle calls priced as "0.0" so if you don't want to charge for the callback this option allows you to rate the callback as 0.0 but still show it.
give it a try. let us know how it works