asterisk users - Jul 2005 - IAX2 ping confusion and unreachable soft phones

I've turned on debug in a (IAXComm based) soft phone. I see the
phone sending pings to *. I see * getting the pings. For some reason,
with iax2 show debug, I never see any response on the console from *.
However, the phone shows a response with INVAL. Seems like an odd
response to a ping request. I believed it should get an ACK. Is that
wrong? If I should get an ACK, what could I have messed up to prevent
* from sending the ACK?

   I also see VNAKs on the * console. I believed that a VNAK basically
meant "you sent me a voice frame at the wrong time." Yet I don't
see
any voice frames being sent before the VNAK.

   I wouldn't be involved with these details, but * keeps deciding
that the phones are unreachable, and I can't figure out why. I believe
I have seen * do that right after a successful (re-) registration with
the phone. It never happens after the first registration: then the
phone is always reachable.
   The phones ping * about every 10 seconds as far as I can tell, and
always get back this INVAL response (and the INVAL never shows up on
the * console).
   The phones are all set up pretty much like this (iax.conf):

[PHONE_NAME]
type=friend
context=inXServices
host=dynamic
secret=************
auth=md5
qualify=1000
mailbox=## <--------- this is really a 2 digit #, different for each phone
language=en
notransfer=yes