There are many things to consider when doing this. You will need a radius server and information from that radius server is needed to configure the tenor. You also have to decide on whether you are going to do post-paid or pre-paid. Pre-paid is usually the way to go with calling cards.
You will need to configure much of this from the tcrg of the tenor as well as the radius interface area.
If you need specific help on this, please contact me directly at email@example.com.