Caller ID for Panasonic systems has always been on a removable daughter card. Many of the past (and all current) systems come from the factory with the cards pre-installed, but none of them really have built-in caller ID. Most of the systems you'll find on the used market have had the cards removed and sold separately, as in many cases the cards alone are worth as much or more than the system itself.
There's no way to make the system have caller ID without installing the card. The only workaround I can think of (and it's really not a good one) is to wire a consumer-grade standalone caller ID unit ahead of the system. This would require significant changes to the customer's wiring and would surely exceed their small budget.
I'd have to double check to be sure, but I've got quite a stockpile of used stuff I've taken on trade-ins and may have one of the 1232 cards and/or a couple of other KSUs with the cards installed. I also know of another place that stocks them regularly....don't think pricing/vendor information is allowed on the open forum but feel free to send me a PM if interested.