asterisk users - Nov 2013 - how to answer a Panasonic PBX extension with Asterisk?

I'd like to have my Asterisk system pick up a certain extension on an
existing Panasonic PBX when it rings.  (It's connected to some
proprietary Panasonic doorphones that I haven't replaced yet.)  I
connected that extension to an FXO port on a Digium AEX410 card, and
set that channel to have the context "doorphone".

The problem is that the extension is never executed.  With debugging
on, I see:

Nov 23 16:37:23] DEBUG[20077]: chan_dahdi.c:11896 do_monitor: Monitor doohicky
got event Ring Begin on channel 12
[Nov 23 16:37:23] DEBUG[20077]: sig_analog.c:3621 analog_handle_init_event:
channel (12) - signaling (5) - event (ANALOG_EVENT_RINGBEGIN)
[Nov 23 16:37:24] DEBUG[20077]: chan_dahdi.c:11896 do_monitor: Monitor doohicky
got event Ring/Answered on channel 12
[Nov 23 16:37:24] DEBUG[20077]: sig_analog.c:3621 analog_handle_init_event:
channel (12) - signaling (5) - event (ANALOG_EVENT_RINGOFFHOOK)
[Nov 23 16:37:29] DEBUG[20077]: chan_dahdi.c:11896 do_monitor: Monitor doohicky
got event Ring Begin on channel 12
[Nov 23 16:37:29] DEBUG[20077]: sig_analog.c:3621 analog_handle_init_event:
channel (12) - signaling (5) - event (ANALOG_EVENT_RINGBEGIN)
[Nov 23 16:37:30] DEBUG[20077]: chan_dahdi.c:11896 do_monitor: Monitor doohicky
got event Ring/Answered on channel 12
[Nov 23 16:37:30] DEBUG[20077]: sig_analog.c:3621 analog_handle_init_event:
channel (12) - signaling (5) - event (ANALOG_EVENT_RINGOFFHOOK)

so it seems to be detecting the ring, but that's it.  What am I missing?

-- 
Eric Cooper             e c c @ c m u . e d u