I have a new install of FreePBX from the x64 iso.
It has 2 x NICs - One for the internal LAN (internal LAN also has access to the external world), One for the External SIP trunk only.
External calls are working as expected.
Internal calls are working as expected.
However, when the users transfer a call (blind or attended) to another internal user, the call drops about 30 seconds after the destination takes the call.
Internal -> Internal calls do not suffer with this issue.
The simple logs are shown below.
Rport settings (in Chan_SIP section) are (No) 30, 300, 0 I have also tested with YES and the same numerical settings.
Please help 
[2015-09-03 10:31:42] VERBOSE[23104][C-0000024b] pbx.c: – Executing [s@macro-dial-one:44] Dial(“SIP/spitfire-000005b4”, “SIP/221,Ttr”) in new stack
[2015-09-03 10:31:42] VERBOSE[23104][C-0000024b] netsock2.c: == Using SIP RTP TOS bits 184
[2015-09-03 10:31:42] VERBOSE[23104][C-0000024b] netsock2.c: == Using SIP RTP CoS mark 5
[2015-09-03 10:31:42] VERBOSE[23104][C-0000024b] app_dial.c: – Called SIP/221
[2015-09-03 10:31:43] VERBOSE[23104][C-0000024b] app_dial.c: – SIP/221-000005b5 is ringing
[2015-09-03 10:31:52] VERBOSE[23104][C-0000024b] app_dial.c: – SIP/221-000005b5 answered SIP/spitfire-000005b4
[2015-09-03 10:32:12] VERBOSE[23104][C-0000024b] pbx.c: – Executing [h@macro-dial-one:1] Macro(“SIP/spitfire-000005b4”, “hangupcall,”) in new stack
[2015-09-03 10:32:12] VERBOSE[23104][C-0000024b] pbx.c: – Executing [s@macro-hangupcall:1] ExecIf(“SIP/spitfire-000005b4”, “0?Set(CDR(recordingfile)=.wav)”) in new stack
[2015-09-03 10:32:12] VERBOSE[23104][C-0000024b] pbx.c: – Executing [s@macro-hangupcall:2] GotoIf(“SIP/spitfire-000005b4”, “1?theend”) in new stack
[2015-09-03 10:32:12] VERBOSE[23104][C-0000024b] pbx.c: – Goto (macro-hangupcall,s,4)
[2015-09-03 10:32:12] VERBOSE[23104][C-0000024b] pbx.c: – Executing [s@macro-hangupcall:4] Hangup(“SIP/spitfire-000005b4”, “”) in new stack
[2015-09-03 10:32:12] VERBOSE[23104][C-0000024b] app_macro.c: == Spawn extension (macro-hangupcall, s, 4) exited non-zero on ‘SIP/spitfire-000005b4’ in macro ‘hangupcall’
[2015-09-03 10:32:12] VERBOSE[23104][C-0000024b] pbx.c: == Spawn extension (macro-dial-one, h, 1) exited non-zero on ‘SIP/spitfire-000005b4’
[2015-09-03 10:32:12] VERBOSE[23104][C-0000024b] app_macro.c: == Spawn extension (macro-dial-one, s, 44) exited non-zero on ‘SIP/spitfire-000005b4’ in macro ‘dial-one’
[2015-09-03 10:32:12] VERBOSE[23104][C-0000024b] app_macro.c: == Spawn extension (macro-exten-vm, s, 16) exited non-zero on ‘SIP/spitfire-000005b4’ in macro ‘exten-vm’
[2015-09-03 10:32:12] VERBOSE[23104][C-0000024b] pbx.c: == Spawn extension (ext-local, 221, 2) exited non-zero on ‘SIP/spitfire-000005b4’