is it over PRI, if so turn on DCH MON in LD 96, look for incoming setup message, from that you can get the TN of the incoming trunk.
To take a look where it is ringing
LD 80
. trad <trunk TN>
To look at config
LD 20 - Print the trunk TN to get the route and member
LD 21 - Print the RDB of the route to get the IDC table
LD 49 - Print the IDC to see where it is going
Hint, often you may not be looking at all the incoming digits, you usually look at the last few controlled by the length of LDN 0 (Print the LDN_DATA in LD 21), so say LDN0 is 1234 then you would be routing in the IDC table based on the last 4 digits