It can be set up by the carrier and checked on your equipment records. Specified Incoming DID calls never get to the switch at all and does a RCR or RCF I believe - Remote Call Redirect or Remote Call Forward of that DID number.
RCR
We have some essential DID numbers assigned, that in the event of a PBX failure and the PBX cannot answer or route to the extension, the number is redirected to a different analog single line phone (not on the PBX but on it's own dedicated line.)
RCF
Assigns the DID to Call Forward to a different number regardless of the PBX status.
KE407122
'Who is this guy named Lo Cel and why does he keep paging me?'