Asterisk Callback

I am looking for someone to make Asterisk A2 Billing **CallBack** work properly. We have Asterisk, A2Billing installed along with live sip trunks and DID's. What we need is from A2Billing signup, account is created. User can then dial to DID, hangup, and then receive a Callback. Caller hears a dialtone and then dials out according to DialPlan. As with all services in A2Billing, user details are recorded. When you submit your bid, please include a URL or method of testing your working sample, or some other method for us to test your work. Not including this work will eliminate you from our consideration.