Our business phone system provides a feature that allows an internal extension number to call another internal extension that's programmed with the user's cell phone number. When you call one to the other, it's supposed to pass through the extension number (i.e. a 3 or 4 digit number) of the caller and pass it on to the cell phone as its Caller-ID.
This works fine with AT&T Wireless. The cell phone user gets the 3 digit extension number as the caller-ID, thus knowing it's an internal user calling. However, on Verizon, the call comes through as "Restricted". Is Verizon not allowing or not able to pass through such short caller-ID numbers?
Sounds like they modified your internal and or external CID database via ATT. However, although what you are doing is for legit reasons, this fringes on a growing issue of caller ID spoofing were organizations tamper with CID or pretend they are a legit business/group. The FCC looks down on this and would probably not take to kindly on what ATT has possibly done to their CID database for your numbers. A phone number should always show up as the officially registered person's name or business (with full phone number) when calling a completely different phone number. If the phones are part of the same account (business or not) this rule still applies.