Telco routing using Asterix (probably!?)

I need to be able to issue telephone numbers which divert to a call answering system which reads the DDI and the CLI and passes this data to my application, which will either return a previously recorded “busy” message OR divert the caller to an available agent. I have little idea what is needed to accomplish this, but I think asterix would be a good start. I also wish to be able to make this work from any country, although would like advice on how this should be achieved. Should it be that the 3rd party telco company with the number ranges reroutes calls to our to be built asterix system or is there a better way. Please include a spec of the hardware needed as well as your personal time and effort to do it.