Check the PRI or SIP trunk group (note this is not possible on T1/CAS or analog trunks), there is a flag called "Propagate Original Caller ID" it must be enabled for this to work, and your carrier must allow it to pass through.\
Note some carriers do not allow an outgoing caller ID number to be sent that isn't a DID of the PRI. Also keep in mind that if the cell number is a LD number to the 5000, the carrier may reject a call that isn't a billable number on the PRI.