gts

Hello all,
I have a customer which has 15 sites with IPO SOE Release 3.0(59) installed.
Most of his site have ISDN BRI lines for incoming and outgoing calls.
Customer complains that suddenly he faced problem with incoming calls to some of his sites.
Sometimes, when we trying to make an incoming call from ISDN BRI we've got message from PTT that "call cannot connect due to technical reasons".
We redial the same number immediatly and the call ringing to the appropriate extension without any problem.
i've setup a monitor trace to one of our SOE, where problem appears and i saw the following message to my BRI lines.

384736mS ISDNL1Evt: v=7 peb=7,F3 F7
384736mS PRN: T4 Started
385486mS ISDNL1Evt: v=7 peb=7,PHDI
385486mS ISDNL3Evt: v=7 p1=7,p2=1001,p3=5,p4=127,s1=
385486mS PRN: T4 Timeout

When i've got this message to a certain line, the next incming call gets error message.
I think that something happen with PTT settings at his interface and especially with Layer1 connectivity(not allways up but only on demand).
Can anyone give me additional information about this message?
Is there something else to look from monitor application in order to verify connectivity issue and ask for a solution ?

Regards