Port goes down on B200M + Asterisk 1.4 + mISDN

My current setup is an Asterisk 1.4.22 with libpri-1.4.8, mISDN-1_1_8 and mISDNuser-1_1_8.
I am using a B200M and experiencing the following symptoms:

When I call in from outside, it always works as expected; no problems at all. A second after I hangup on a call initiated from outside, the stacks look like this:

*CLI> misdn show stacks
BEGIN STACK_LIST:
* Port 1 Type TE Prot. PTP L2Link UP L1Link:UP Blocked:0 Debug:0


Everything is fine, I can dial out as well now without problems.
But after a certain period of time, usually around 20 seconds, I cannot dial out anymore. There is an error message indicating that the port is down and no line available.

The stacks now look like this:

*CLI> misdn show stacks
BEGIN STACK_LIST:
* Port 1 Type TE Prot. PTP L2Link DOWN L1Link:DOWN Blocked:0 Debug:0

Even Layer 1 goes down, the cable still plugged in of course. When i repeat dialing in from outside, the port goes up again, Layer 1 and 2.

Is there a particular setting I am missing in Asterisk to keep these ports UP permanently or is this an mISDN issue? Glad for any help

Regards
Stefan

hello,
i think you are facing the problem like this:
http://misdn.org/index.php
/FAQ_chan_mISDN#Why_does_the_L1_goes_DOWN_on_my_PMP_Isdn_Link.3F_Or_why_do_i_get_No_free_chan_even_after_group_call_from_chan_misdn_if_dialing_out_on_my_PMP_Link.3F
please check that!
james.zhu