Thanks for posting the link.
I really fancy building a system here at home using vintage phones but need a PABX and a couple more phones too.
Keep an eye out for a BT "2=8" system.. These can be used as a 'key system' with the dedicated phones (like rocking horse droppings) or can be used with POTs (Plain Ordinary Telephones) normal GPO/BT telephones, to me and you.. I got mine for about three quid, if memory serves me, and can be connected to your incoming telephone line (DEL) so that you can get all the benefits of an internal telephone system, and can dial direct to and through the BT exchange.
I got mine from York Auction Centre, which was not the best place for the seller to offer it up.. It would have made much, much more on the bay.
Other systems that can be had quite cheaply are the older (1980's) SDX 'Sterling' systems, SDX-10 etc, though these use dedicated phone (which are quite easy to get, and often cheaply).. Panasonic made an excellent system too, and I do believe Chris has one of these, but the phones are not readily available, so it's worth bearing in mind, if you were to buy such a system, makes sure it has enough hansets (telephones) to meet your needs, though that would be good advice with any system - other than the BT "2+8" which as I said earlier, works with ordinary phones (DTMF of loop disconnect) which makes it a rather sought after system.