And here is the log, collected from FreeSWITCH console:
-------------------------------------------------------------------------------
send 1295 bytes to tcp/[IP.ADR.WAN.OBI]:29529 at 22:06:04.579657:
------------------------------------------------------------------------
SIP/2.0 200 OK
Via: SIP/2.0/TCP IP.ADR.LAN.FS:4276;branch=z9hG4bK-5e08f5b0;rport=29529;received=IP.ADR.WAN.OBI
From: "OBi SP1" <sip:2005@domain.tld>;tag=SP13bdf1fbd4cf1dec5
To: <sip:2001@domain.tld>;tag=064Ka82rNDp4N
Call-ID: 64ddfbe7@IP.ADR.LAN.FS
CSeq: 8002 INVITE
Contact: <sip:2001@IP.ADR.WAN.FS:5070;transport=tcp>
User-Agent: FreeSWITCH-mod_sofia/1.3.17+git~20130310T201453Z~05895faa77
Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, INFO, UPDATE, REGISTER, REFER, NOTIFY, PUBLISH, SUBSCRIBE
Supported: timer, precondition, path, replaces
Allow-Events: talk, hold, conference, presence, dialog, line-seize, call-info, sla, include-session-description, presence.winfo, message-summary, refer
Session-Expires: 120;refresher=uas
Min-SE: 120
Content-Type: application/sdp
Content-Disposition: session
Content-Length: 365
Remote-Party-ID: "Outbound Call" <sip:2001@domain.tld>;party=calling;privacy=off;screen=no
v=0
o=- 3572719548 3572719549 IN IP4 IP.ADR.WAN.CSS
s=pjmedia
c=IN IP4 IP.ADR.WAN.CSS
t=0 0
m=audio 4014 RTP/AVP 18 101
c=IN IP4 IP.ADR.WAN.CSS
a=rtpmap:18 G729/8000
a=fmtp:18 annexb=no
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
a=rtcp:4015 IN IP4 IP.ADR.WAN.CSS
a=zrtp-hash:1.10 1700947ed37d7a37c3c94a83da6b441a80d77dccea7fcc0c113a8d6f72d2aa7f
------------------------------------------------------------------------
recv 593 bytes from udp/[IP.ADR.WAN.OBI]:10135 at 22:06:04.609702:
------------------------------------------------------------------------
ACK sip:2001@IP.ADR.WAN.FS:5070;transport=tcp SIP/2.0
Call-ID: 64ddfbe7@IP.ADR.LAN.FS
Content-Length: 0
CSeq: 8002 ACK
From: "OBi SP1" <sip:2005@domain.tld>;tag=SP13bdf1fbd4cf1dec5
Max-Forwards: 70
To: <sip:2001@domain.tld>;tag=064Ka82rNDp4N
Via: SIP/2.0/TCP IP.ADR.LAN.FS:4276;branch=z9hG4bK-1b6f07d6;rport
Proxy-Authorization: DIGEST algorithm=MD5,nc=00000001,qop=auth,cnonce="7cbd1fe1110ec270",nonce="a3f6eae2-364f-42d6-974e-9cc2171e5524",realm="domain.tld",response="2689629580d2ff2dd34b031294a16e57",uri="sip:2001@domain.tld:5070",username="2005"
User-Agent: OBIHAI/OBi100-1.3.0.2774
-------------------------------------------------------------------------------
In the log I've replaced:
IP.ADR.WAN.OBI
IP.ADR.WAN.FS
IP.ADR.WAN.CSS
IP.ADR.LAN.FS
domain.tld
The rest, including port numbers, transport etc. is untouched.
As you can see SIP server sends INVITE/200 OK via TCP transport, while OBi replies ACK via UDP, braking call in next 30 sec.