Not sure what system you have, so I don't know if you have the same options that I do, but, this is what I am doing -
I have an MXeIII contoller and we use CPN substitution for Caller ID info. There are 2 places to configure this - DID Ranges for CPN Substitution where you set up the internal extension to External number mapping and then CPN Substitution where you add the index numbers from DID Ranges. We have 2 SIP trunks coming in, so I have to add the indexes on both trunks in this form. YMMV.