asterisk users - Nov 2010 - help with annoying warning message: Asked to transmit frame type ulaw, while native formats is 0x2 (gsm) read/write = 0x2 (gsm)/0x2 (gsm)

Hi all,
i have a little problem to understand this warning message, it's annoying
and it cause a lot of spurious in the log files.
Im working with this scenario:

a sip outgoing trunk (Trunk-out) that support only ulaw and all calls are
always routed to this.
a list of sip UAs that potentially can use any codec apart g729/g723.
I setup the asterisk to do as mediaproxy so directmedia=no and
canreinvite=no. I want the rtp traffic goes through asterisk.

I can reproduce the waring message below when a peer uses a different codec
that ulaw.
when the UA send out a number to route through the trunk, the warning
message is not displayed initially, but only when the ring tone starts,
then the mesage appears repeatedly and stops when the called peer answers,
and the call is bridged successfully without problems.

Im using asterisk 1.8.0
from sip.conf

[Trunk-out]
type=peer
host=*.*.*.*
context=from-Trunk-out
insecure=port,invite
qualify=yes
disallow=all
allow=ulaw
directmedia=no
canreinvite=no
dtmfmode=rfc2833

[ua1]
type=friend
secret=*
host=dynamic
nat=yes
qualify=60000
disallow=all
allow=ulaw
allow=gsm
allow=ilbc
allow=alaw
qualifyfreq=9
context=sswtrunks
directmedia=no
canreinvite=no

[ua2]
...



  == Using SIP RTP CoS mark 5
    -- Executing [***********@sswtrunks:1] Dial("SIP/ua1-00000776",
"SIP/Trunk-out/***************,180,tTr") in new stack
  == Using SIP RTP CoS mark 5
    -- Called Trunk-out/******************
    -- SIP/Trunk-out-00000777 is ringing
    -- SIP/Trunk-out-00000777 is making progress passing it to
SIP/ua1-00000776
[Nov 19 17:08:37] WARNING[8377]: chan_sip.c:6027 sip_write: Asked to
transmit frame type ulaw, while native formats is 0x2 (gsm) read/write = 0x2
(gsm)/0x2 (gsm)
[Nov 19 17:08:37] WARNING[8377]: chan_sip.c:6027 sip_write: Asked to
transmit frame type ulaw, while native formats is 0x2 (gsm) read/write = 0x2
(gsm)/0x2 (gsm)
[Nov 19 17:08:37] WARNING[8377]: chan_sip.c:6027 sip_write: Asked to
transmit frame type ulaw, while native formats is 0x2 (gsm) read/write = 0x2
(gsm)/0x2 (gsm)
[Nov 19 17:08:37] WARNING[8377]: chan_sip.c:6027 sip_write: Asked to
transmit frame type ulaw, while native formats is 0x2 (gsm) read/write = 0x2
(gsm)/0x2 (gsm)
[Nov 19 17:08:37] WARNING[8377]: chan_sip.c:6027 sip_write: Asked to
transmit frame type ulaw, while native formats is 0x2 (gsm) read/write = 0x2
(gsm)/0x2 (gsm)
[Nov 19 17:08:37] WARNING[8377]: chan_sip.c:6027 sip_write: Asked to
transmit frame type ulaw, while native formats is 0x2 (gsm) read/write = 0x2
(gsm)/0x2 (gsm)
[Nov 19 17:08:37] WARNING[8377]: chan_sip.c:6027 sip_write: Asked to
transmit frame type ulaw, while native formats is 0x2 (gsm) read/write = 0x2
(gsm)/0x2 (gsm)
[Nov 19 17:08:37] WARNING[8377]: chan_sip.c:6027 sip_write: Asked to
transmit frame type ulaw, while native formats is 0x2 (gsm) read/write = 0x2
(gsm)/0x2 (gsm)

over and over....

[Nov 19 17:08:38] WARNING[8377]: chan_sip.c:6027 sip_write: Asked to
transmit frame type ulaw, while native formats is 0x2 (gsm) read/write = 0x2
(gsm)/0x2 (gsm)
    -- SIP/Trunk-out-00000777 answered SIP/ua1-00000776
  == Spawn extension (sswtrunks, ********, 1) exited non-zero on
'SIP/ua1-00000776'

any idea ??
thanks in advance
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