Incoming IAX calls show Anonymous instead of extension number

I have a customer that has 6 locations. They all have IAX trunks to call each other. When one particular location calls another, (and it is the same two) the called party gets Anonymous as the caller info. All the systems are at 10.13.66-17. Both trunks are set for inter-company in the setup. All the extensions have the CID as blank.