asterisk users - Aug 2007 - WARNING[26091]: chan_sip.c:9835 handle_response_register: Got 200 OK on REGISTER that isn't a register

hi ppl, 
i get this error in my asterisk CLI:

Aug 31 02:45:31 WARNING[26091]: chan_sip.c:9835 handle_response_register: Got
200 OK on REGISTER
that isn't a register
Aug 31 10:22:24 WARNING[26091]: chan_sip.c:9835 handle_response_register: Got
200 OK on REGISTER
that isn't a register

I guess it is related to my problem i have with one of my voip providers:
i'm using asterisk 1.2 because i'm making use of voiceone, but before i
was using asterisk
1.4 and had the same problem, it concerns an italian voip/sip provider called
eutelia/skypho, my
problem is the following one:
when i start my pbx my skypho account is working fine, meaning that e.g.
incoming calls are shown
in the asterisk CLI and caller and callee can hear each other when picked up,
but after a while it
stops working, incoming calls for this provider are not shown anymore in the
CLI, but from other
providers it always works, but the phone is ringingn nevertheless when calling
my skypho
account...when i then turn off the pbx and restart after sumthing like 2 hours
my skypho account
is working fine again, the incmiong calls are shown in the asterisk CLI, but
after, i don't know
let's say an hour or so it again stops working, incoming calls for my skypho
account can not be
seen in the asterisk CLI, then if i turn off the pbx for an hour or so it works
again, so i
thought it must be a setting issue, maybe something with the register? althought
it always shows
it registered when i use 'sip show registry' someone has an idea what i
have to set or do to have
it working permanently? I tried qualify on and off, but that did not help...what
could be the
problem here? i got no clue whatsoever and i have been
using asterisk only since half a year, please help me, i'm totaly desperate,
thx in advance

some more info about my system
debian etch 4.0  
kernel: 
2.6.18-4-686 
Asterisk 1.2.13 
I use VoiceOne (www.voiceone.it) version is v. 0.5.0 using plugin subsystem v.
0.4pre3
mysql Ver 14.12 Distrib 5.0.32, for pc-linux-gnu (i486) using readline 5.2 
PHP 4.4.4-8+etch4 (cli) (built: Jun 30 2007 21:02:54) 
using grandstream(handytone) 486 as sip device, no other devices or PSTN
connected, only using
sip/voip providers 
behind router/NAT


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