New Tech Guy,
By default the 5000 will accept any and all calls regardless of Caller ID being withheld or not. More than likely someone set it up to not allow them and that would be under the Call Routing Tables.
Typically CRT are routing according to the DID or DDI sent to the system, labeled as Trunk Number, but it can be changed to look at the Caller ID to make a decision. So, look for a CRT that routes according to Outside Party Number and that will be your culprit.
Thanks,
TE