Incoming Routes of a External caller that was Call Forwarded

I’m attempting to using fpbx 2.8 incoming routing to handled this scenario.

We have a user with an internal extension that is usually out-of-the-office. The user will most often be on a cellular connection and wants to call forward that connection to the office fpbx and to reach through to his voice mail. Currently, the office has two DIDs.

When his phone is forwarded to the office, Are there any methods available to determine the Incoming Call originated from his cellular number? I’ve attempted to define both CID and DID using his cellular number without success.

Any suggestions are appreciated.