They'll just be well-hidden - probably somewhere behind the custom-fitted, polished, solid-walnut cabinets in the front office...
To terminate your analogue lines, you will somewhere have BT sockets with normal phone connectors - the BT engineers I spoke to said that is always the case. So, yes, if you have 12 analogue lines you'd have 12 normal phone sockets.
How many lines do you have? You implied at the start of this thread that you were okay with the analouge lines you have - have those companies mentioned above changed your mind, or do we just have a larger amount of telephone lines?
However, I'd suggest that with that number of lines, you should be looking at replacing them with ISDN or IAX/SIP over your net connection.