asterisk users - May 2017 - connecting two asterisks - transfer=no

I'm using Asterisk 11 and have a problem with when making call transfer on
remote Asterisk.

This dial plan below works when I make a call directly to remote asterisk
dialing FXO on remote asterisk.

exten => 4,1,Dial(${FD_L1},25,trw)
exten =>
4,n,GotoIf($["${DIALSTATUS}"="BUSY"]?vmail:line2)
exten => 4,n(line2),Dial(${FD_L2},20,rw)
exten => 4,n(vmail),Voicemail(4)
exten => 4,n,Voicemail(4)
exten => 4,n,Hangup()

Line one FD_L1 rings for 25sec. nobody pickup the line so FD_L2 rings for 20sec
and if nobody pickup the line it goes to voice mail.

However if I make a call over VPN to remote Asterisk, and dial exten: 4
FD_L1 rings for 25sec. and as soon as the call gets transferred to FD_L2 it gets
abruptly terminated with a message:

  exited non-zero on 'IAX2

 
-- SIP/54-00000006 is ringing
== Spawn extension (extensions, 4, 3) exited non-zero on
'IAX2/home_server-424'

Does it have something to do with "transfer=no" in iax.conf?

-- 
Thelma