but anyway, siptrace does not writing to log, only to fs_cli terminal output.

Anyway, I caught SIPtrace messages for Nortel, this is piece of terminal output.
I see, FS sends signal BYE, because ACK timeout:

2011-11-02 18:32:07.929071 [DEBUG] switch_ivr_play_say.c:1672 done playing file /usr/local/freeswitch/sounds/ru/RU/elena/digits/1.wav
recv 330 bytes from udp/[192.168.1.17]:5061 at 22:32:07.995778:
   ------------------------------------------------------------------------
   NOTIFY sip:192.168.1.5 SIP/2.0
   Via: SIP/2.0/UDP 192.168.1.17:5061;branch=z9hG4bK-c51ee6c4
   From: 1004 <sip:1004@192.168.1.5>;tag=12dc6681765b973do1
   To: <sip:192.168.1.5>
   Call-ID: af69bdb1-3c02a9cd@192.168.1.17
   CSeq: 274133 NOTIFY
   Max-Forwards: 70
   Event: keep-alive
   User-Agent: Linksys/PAP2-3.1.22(LS)
   Content-Length: 0

   ------------------------------------------------------------------------
send 669 bytes to udp/[192.168.1.17]:5061 at 22:32:08.001770:
   ------------------------------------------------------------------------
   SIP/2.0 200 OK
   Via: SIP/2.0/UDP 192.168.1.17:5061;branch=z9hG4bK-c51ee6c4
   From: 1004 <sip:1004@192.168.1.5>;tag=12dc6681765b973do1
   To: <sip:192.168.1.5>;tag=c0N5gH4XH8DQm
   Call-ID: af69bdb1-3c02a9cd@192.168.1.17
   CSeq: 274133 NOTIFY
   Contact: <sip:173.79.237.112>
   User-Agent: FreeSWITCH-mod_sofia/1.0.head-git-f847059 2011-10-28 15-58-17 -0500
   Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, UPDATE, INFO, REGISTER, REFER, NOTIFY, PUBLISH, SUBSCRIBE
   Supported: timer, precondition, path, replaces
   Allow-Events: talk, hold, presence, dialog, line-seize, call-info, sla, include-session-description, presence.winfo, message-summary, refer
   Content-Length: 0

   ------------------------------------------------------------------------
2011-11-02 18:32:08.195841 [DEBUG] switch_ivr_play_say.c:244 Handle play-file:[ivr/ivr-to_do_a_freeswitch_echo_test.wav] (ru:ru)
2011-11-02 18:32:08.195841 [DEBUG] switch_ivr_play_say.c:1302 Codec Activated L16@8000hz 1 channels 20ms
send 1154 bytes to udp/[192.168.1.158]:5060 at 22:32:09.084079:
   ------------------------------------------------------------------------
   SIP/2.0 200 OK
   Via: SIP/2.0/UDP 192.168.1.158:5060;branch=z9hG4bK-6e-1ae15-7e234642
   From: "1000"<sip:1000@olegh.ath.cx>;tag=2f5c50-9e01a8c0-13c4-45026-6d-ad711f2-6d
   To: <sip:5000@olegh.ath.cx>;tag=1vZmX7aQH6r9m
   Call-ID: 3039a8-9e01a8c0-13c4-45026-6d-1a07e714-6d
   CSeq: 2 INVITE
   Contact: <sip:5000@173.79.237.112:5060;transport=udp>
   User-Agent: FreeSWITCH-mod_sofia/1.0.head-git-f847059 2011-10-28 15-58-17 -0500
   Accept: application/sdp
   Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, UPDATE, INFO, REGISTER, REFER, NOTIFY, PUBLISH, SUBSCRIBE
   Supported: timer, precondition, path, replaces
   Allow-Events: talk, hold, presence, dialog, line-seize, call-info, sla, include-session-description, presence.winfo, message-summary, refer
   Content-Type: application/sdp
   Content-Disposition: session
   Content-Length: 242
   Remote-Party-ID: "5000" <sip:5000@olegh.ath.cx>;party=calling;privacy=off;screen=no

   v=0
   o=FreeSWITCH 1320248473 1320248474 IN IP4 192.168.1.5
   s=FreeSWITCH
   c=IN IP4 192.168.1.5
   t=0 0
   m=audio 24624 RTP/AVP 0 99
   a=rtpmap:0 PCMU/8000
   a=rtpmap:99 telephone-event/8000
   a=fmtp:99 0-16
   a=silenceSupp:off - - - -
   a=ptime:20
   ------------------------------------------------------------------------
2011-11-02 18:32:09.554915 [DEBUG] switch_core_session.c:872 Send signal sofia/internal/1000@olegh.ath.cx [BREAK]
send 685 bytes to udp/[192.168.1.158]:5060 at 22:32:09.573772:
   ------------------------------------------------------------------------
   BYE sip:1000@192.168.1.158:5060 SIP/2.0
   Via: SIP/2.0/UDP 173.79.237.112;rport;branch=z9hG4bKmBgZ4SN8yDSag
   Max-Forwards: 70
   From: <sip:5000@olegh.ath.cx>;tag=1vZmX7aQH6r9m
   To: "1000" <sip:1000@olegh.ath.cx>;tag=2f5c50-9e01a8c0-13c4-45026-6d-ad711f2-6d
   Call-ID: 3039a8-9e01a8c0-13c4-45026-6d-1a07e714-6d
   CSeq: 19800500 BYE
   Contact: <sip:5000@173.79.237.112:5060;transport=udp>
   User-Agent: FreeSWITCH-mod_sofia/1.0.head-git-f847059 2011-10-28 15-58-17 -0500
   Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, UPDATE, INFO, REGISTER, REFER, NOTIFY, PUBLISH, SUBSCRIBE
   Supported: timer, precondition, path, replaces
   Reason: SIP;cause=408;text="ACK Timeout"
   Content-Length: 0

   ------------------------------------------------------------------------
2011-11-02 18:32:09.554915 [DEBUG] switch_core_session.c:872 Send signal sofia/internal/1000@olegh.ath.cx [BREAK]
recv 479 bytes from udp/[192.168.1.158]:5060 at 22:32:09.586581:
   ------------------------------------------------------------------------
   SIP/2.0 200 OK
   From: <sip:5000@olegh.ath.cx>;tag=1vZmX7aQH6r9m
   To: "1000"<sip:1000@olegh.ath.cx>;tag=2f5c50-9e01a8c0-13c4-45026-6d-ad711f2-6d
   Call-ID: 3039a8-9e01a8c0-13c4-45026-6d-1a07e714-6d
   CSeq: 19800500 BYE
   Via: SIP/2.0/UDP 173.79.237.112;received=192.168.1.5;rport=5060;branch=z9hG4bKmBgZ4SN8yDSag
   Supported: replaces
   Allow: INVITE,ACK,BYE,CANCEL,OPTIONS,INFO,SUBSCRIBE,NOTIFY,PRACK,UPDATE,REFER
   User-Agent: Nortel IP Phone 1535 (0.2.50.0905)
   Content-Length: 0

   ------------------------------------------------------------------------
2011-11-02 18:32:09.582519 [DEBUG] switch_core_session.c:872 Send signal sofia/internal/1000@olegh.ath.cx [BREAK]
2011-11-02 18:32:09.582519 [DEBUG] sofia.c:5283 Channel sofia/internal/1000@olegh.ath.cx entering state [terminating][0]
2011-11-02 18:32:09.582519 [DEBUG] switch_channel.c:2804 (sofia/internal/1000@olegh.ath.cx) Callstate Change ACTIVE -> HANGUP
2011-11-02 18:32:09.582519 [NOTICE] sofia.c:6039 Hangup sofia/internal/1000@olegh.ath.cx [CS_EXECUTE] [NORMAL_UNSPECIFIED]