I have taken my final year Electrical BE project on "Designing Cost Effective PBX System" for two Co lines and 8 Extensions. Iam using the Mitel SLIC Chips and Mitel DTMF decoders.Working on the software part is quite difficult process. Any website for such issues of programming telephone exchanges..i have searched this forum and found lots of posts but none have any replies of satisfaction. the software includes scanning of ports after every 10 milli second interrupt, considering the previous port condidtions then dailing processs and then sending the call, the feautres problem is there..what are the simplest features that should be placed in this PBX. Like Voice Messages when some one call at this PBX , the message should be able to guide the caller to different extension which he can then dial to directly go to that extension , this feature is called DISA direct inward System Access.is it possible using 89c52 or any chip is required to be interfaced..