we had call-limit=1 working on our test box running 1.2.1, once patch 6111 was applied...that has been added to the source and is present in 1.2.4, as far as i can tell.
however, 1.2.4 is broken now, as it is continually setting the device state 1 (not in use). I don't see anything obvious in the source code, but then again, I'm not a programmer by trade.
but, after the patch in bug 6111 was applied, call-limit WAS working properly, and I would be upgrading all of our servers to 1.2.4 if it was working still.
EDIT: the fix is to change type=friend to type=peer in sip.conf. All of a sudden, call-limit works again. What else this might affect, I do not know, but hopefully someone with more expertise might.
this was on version 1.2.1, i'm trying 1.2.4 shortly (as soon as it's done compiling)
EDIT AGAIN: same on 1.2.4 - type=friend allows any number of calls through, while type=peer works properly with call-limit=1 (or 2, or whatever).