Ok. Finally fixed.

I had to add the following option to get rid off the problem above.

1. insecure = invite

So finally my config for mynetfone trunk_1 looks like:

[trunk_1]
allow = g729
context = DID_trunk_1
dialformat = ${EXTEN:1}
canreinvite = yes
hasexten = no
hasiax = no
hassip = yes
host = sip00.mynetfone.com.au
port = 5060
registeriax = no
registersip = yes
secret = password
trunkname = Custom – mynetfone
trunkstyle = customvoip
username = mynetfone_sip_username
dtmf = rfc2833
fromuser = mynetfone_sip_username
callerid = DID
type = friend
insecure = invite