From FreeSWITCH UA to FreeSWITCH Server:

10.250.5.17:5090 -> 10.250.5.50:5090
  INVITE sip:185511@192.168.1.30 SIP/2.0
  Via: SIP/2.0/UDP 10.250.5.17:5090;branch=z9hG4bK.I4KG97dOK;rport
  From: <sip:1001@10.250.5.50>;tag=MeYxCN9Lq
  To: sip:185511@192.168.1.30
  CSeq: 20 INVITE..Call-ID: t2XYFvCyYU..Max-Forwards: 70
  Route: <sip:10.250.5.50:5090;lr
  Supported: outbound
  Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY, MESSAGE, SUBSCRIBE, INFO, UPDATE
  Content-Type: application/sdp
  Content-Length: 328
  Contact: <sip:1001@10.250.5.17:5090>;+sip.instance="<urn:uuid:2f0b19de-2db0-4bef-be7d-48bf483a8141>"
  User-Agent: Linphone/3.9.1 (belle-sip/1.4.2)
  v=0
  o=1001 36791124 IN IP4 10.250.5.17
  s=Talk
  c=IN IP4 10.250.5.17
  t=0 0
  a=rtcp-xr:rcvr-rtt=all:10000 stat-summary=loss,dup,jitt,TTL voip-metrics
  m=audio 7078 RTP/AVP 0 8 101
  a=rtpmap:101 telephone-event/8000
  m=video 9078 RTP/AVP 9697
  a=rtpmap:96 VP8/90000
  a=rtpmap:97 H264/90000
  a=fmtp:97 profile-level-
  id=42801F

10.250.5.50:5090 -> 10.250.5.17:5090
  SIP/2.0 100 Trying
  Via: SIP/2.0/UDP 10.250.5.17:5090;branch=z9hG4bK.I4KG97dOK;rport=5090
  From: <sip:1001@10.250.5.50>;tag=MeYxCN9Lq
  To: <sip:185511@192.168.1.30>
  Call-ID: t2XYFvCyYU
  CSeq: 20 INVITE
  User-Agent: FreeSWITCH-mod_sofia/1.7.0+git~20151211T002348Z~cae255395f~64bit
  Content-Length: 0

From FreeSWITCH Server to OpenSIPS:

10.250.5.50:5090 -> 10.250.5.3:5080
  INVITE sip:185511@10.250.5.3:5080 SIP/2.0
  Via: SIP/2.0/UDP 10.250.5.50:5090;rport;branch=z9hG4bKQ4XSKc0vUp4FD Max-Forwards: 69
  From: "1001" <sip:1001@[10.250.5.50:5090]>;tag=2XyUKHFD2y6pF
  To: <sip:185511@10.250.5.3:5080>
  Call-ID: 65474177-6471-1234-e187-000c295a647b
  CSeq: 88639388 INVITE
  Contact: <sip:gw+osips@10.250.5.50:5090;transport=udp;gw=osips>
  User-Agent: FreeSWITCH-mod_sofia/1.7.0+git~20151211T002348Z~cae255395f~64bit
  Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, INFO, UPDATE, REGISTER, REFER, NOTIFY
  Supported: timer, path, replaces
  Allow-Events: talk, hold, conference, refer
  Content-Type: application/sdp
  Content-Disposition: session
  Content-Length: 218
  X-FS-Support: update_display,send_info
  Remote-Party-ID: "1001" <sip:1001@10.250.5.50:5090>;party=calling;screen=yes;privacy=off
  v=0
  o=FreeSWITCH 1457928161 1457928162 IN IP4 10.250.5.50
  s=FreeSWITCH
  c=IN IP4 10.250.5.50
  t=0 0
  m=audio 22744 RTP/AVP 8 101
  a=rtpmap:8 PCMA/8000
  a=rtpmap:101 telephone-event/8000
  a=fmtp:101 0-16
  a=ptime:20

U 10.250.5.3:5080 -> 10.250.5.50:5090
  SIP/2.0 100 Giving a try
  Via: SIP/2.0/UDP 10.250.5.50:5090;received=10.250.5.50;rport=5090;branch=z9hG4bKQ4XSKc0vUp4FD
  From: "1001" <sip:1001@[10.250.5.50:5090]>;tag=2XyUKHFD2y6pF
  To: <sip:185511@10.250.5.3:5080>
  Call-ID: 65474177-6471-1234-e187-000c295a647b
  CSeq: 88639388 INVITE
  Server: OpenSIPS (2.1.2 (x86_64/linux))
  Content-Length: 0

U 10.250.5.3:5080 -> 10.250.5.50:5090
  SIP/2.0 400 Bad Request
  Warning: 399 192.168.1.30 "Malformed headers : From "
  To: <sip:185511@10.250.5.3:5080>;tag=7ea5fd275b6bfe2bcdd1565b6841ecc6
  Call-ID: 65474177-6471-1234-e187-000c295a647b
  CSeq: 88639388 INVITE
  Via: SIP/2.0/UDP 10.250.5.50:5090;received=10.250.5.50;rport=5090;branch=z9hG4bKQ4XSKc0vUp4FD
  Content-Length: 0

After that only timeout or cancels ends sending this error.