Hrmm the configuration of the Cisco Gateway isn’t that hard but the writing of the script to handle the prompts is the job of an experianced VOIP programmer. It’s not easy at all. There are some example scripts to load to your Gateway at Cisco’s web site, but they don’t work without heavy modification. I would suggest that to do what you want you’re going to have to hire or contract to an experianced VOIP engineer.
Sorry I can’t give you better news. But I’ve been doing VOIP for 2 years and It’s still not the easiest thing to impliment a Calling card service from scratch.
The configuration I gave you above would do your Gateway but the script and voice announcement files are another matter.