Step 1 is to find out what API the switch uses. Does it support TAPI?

>I need to write one program which can take the caling numbers from each extension from PBX. (we have Nortel EPBX). How do i start with it?? I sould refere TAPI controls to do this?? Is it the right way?? can anyone please guide to me to find any sample code or references.
>thanks and regards