IDC is Incoming Digit Conversion. Basically it translates an incoming DDI number to your CDN.
If, like us, you're using a 1:1 relationship between DDI and CDN i.e. 01XXXX50000 becomes CDN 50000, then you don't need it, however, when using DNIS, you translate several incoming DDI to one CDN
01XXXX50001 is translated to 50000 but also
01XXXX50002 is translated to 50000 and
01XXXX50003 is translated to 50000 etc. etc
Each of the above numbers would route to the same CDN, but have different DNIS.
We have 2 x 10,000 number ranges (50000 and 60000), so we would need to have five digits of DNIS to ensure no duplicate DNIS (if we had only 4 digits, then 01XXXX50001 and 01XXXX60001 would both have the same DNIS)
The IDC table is pegged against the incoming route(s).
Use ld 21 to print the route and verify that IDC is YES and what the table number is (DCNO). Then go to ld 49 to print the relevant table DCNO. This will give you the incoming digits.
Printing the route in ld 21 will also tell you if DNIS is enabled (DNIS = YES) and how many digits are used (NDGT = 5 for example)
If you're setting it up, you'll need to build the IDC table first and then associate it to the route and then set up the DNIS digits.
DD