mISDN problems

Hi,
I use a b100p isdn openvox card on my txb 2.8.0.3 installation.
When operators use the attended transfer (default *2) on their spa922 cisco phones, when the first hangs up, the second loses the comunication with the external incoming call. The operator can hear the other side but the external user hears nothing. On the other hand, attended transfer seems to work fine.
And worse, after some time of operating trixbox and quite randomly, the "all circuits are busy" message is activated and no calls can be placed untill rebooting. And that's frustrating.

Here are some parts of the misdn.log:
Thu Jun 10 15:54:23 2010: P[ 1] Got Unsupported Frame with Format:4
Thu Jun 10 15:54:23 2010: P[ 1] Got Unsupported Frame with Format:4
Thu Jun 10 15:54:23 2010: P[ 1] Got Unsupported Frame with Format:4
Thu Jun 10 15:54:23 2010: P[ 1] Got Unsupported Frame with Format:4
Thu Jun 10 15:54:23 2010: P[ 1] Got Unsupported Frame with Format:4
Thu Jun 10 15:54:23 2010: P[ 1] Got Unsupported Frame with Format:4
Thu Jun 10 15:54:23 2010: P[ 1] Got Unsupported Frame with Format:4
Thu Jun 10 15:54:23 2010: P[ 1] Got Unsupported Frame with Format:4
Thu Jun 10 15:54:23 2010: P[ 1] Got Unsupported Frame with Format:4
Thu Jun 10 15:54:23 2010: P[ 1] Got Unsupported Frame with Format:4
Thu Jun 10 15:54:23 2010: P[ 1] Got Unsupported Frame with Format:4
Thu Jun 10 15:54:23 2010: P[ 1] Got Unsupported Frame with Format:4
Thu Jun 10 15:54:23 2010: P[ 1] Got Unsupported Frame with Format:4
Thu Jun 10 15:54:23 2010: P[ 1] Got Unsupported Frame with Format:4
Thu Jun 10 15:54:23 2010: P[ 1] Got Unsupported Frame with Format:4
Thu Jun 10 15:54:23 2010: P[ 1] Got Unsupported Frame with Format:4
Thu Jun 10 15:54:23 2010: P[ 1] Got Unsupported Frame with Format:4

And this goes on and on for many times in a second and for some minutes.

I also get this:

Thu Jun 10 10:05:24 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:05:43 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:06:02 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:07:08 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:08:13 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:08:49 2010: P[ 1] --> we have already send Release_complete
Thu Jun 10 10:08:49 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:12:07 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:19:02 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:19:27 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:20:01 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:26:37 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:26:51 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:27:47 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:29:08 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:29:49 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:29:49 2010: P[ 1] --> we have already send Release_complete
Thu Jun 10 10:30:13 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:30:13 2010: P[ 1] --> we have already send Release_complete
Thu Jun 10 10:30:41 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:30:41 2010: P[ 1] CC_RELEASE_COMPLETE|CONFIRM [TE]
Thu Jun 10 10:37:03 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:41:44 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:42:05 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 10:44:43 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 12:16:53 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 12:38:45 2010: P[ 1] empty_chan_in_stack: cannot empty channel 255
Thu Jun 10 12:38:45 2010: P[ 1] --> we have already send Release_complete

Here is my misdn.conf.

context=misdn
language=it
musicclass=default
senddtmf=yes
far_alerting=no
allowed_bearers=all
nationalprefix=0
internationalprefix=00
rxgain=0
txgain=0
te_choose_channel=no
pmp_l1_check=no
reject_cause=16
need_more_infos=no
nttimeout=no
method=standard
dialplan=0
localdialplan=0
cpndialplan=0
early_bconnect=yes
incoming_early_audio=no
nodialtone=no
callgroup=1
pickupgroup=1
presentation=-1
screen=-1
echocancel=yes
jitterbuffer=4000
jitterbuffer_upper_threshold=0
hdlc=no

[isdn]
ports=1
context=from-trunk
msns=*

Thank you for any support you might provide me with.