For every stanza in iax.conf add or change codecpriority=host. That forces the server to set the ordered list.
A while back I had a separate issue where ClearNode users could not successfully negotiate a connection to my hub node, so I switched [radio] to codecpriority=caller, which solved that issue.
I would prefer to leave my hub node to caller, then have the other nodes set their codec priority accordingly, which is what led me to this concern.
I just realized that my [radio] stanza is actually pointing to the radio-secure context, so I'm guessing it won't help to create a new stanza called radio-secure. Or would it?
; Incoming radio connections
[radio]
type = user
disallow = all
;allow = slin16
;allow = slin
allow = ulaw
allow = adpcm
allow = g722
allow = g726aal2
allow = gsm
allow = ilbc
codecpriority = caller
context = radio-secure
transfer = no
[iaxrpt] ; Connect from iaxrpt Username field (PC AllStar Client)
type = user ; Notice type is user here <---------------
context = iaxrpt ; Context to jump to in extensions.conf
auth = md5
secret = Motorola1rola1
host = dynamic
*disallow = all *
allow = ulaw
allow = adpcm
*allow = gsm *
transfer = no
[phone-iaxrpt]
type = friend
context = phone-iaxrpt
auth = md5
secret = Motorola1rola1
host = dynamic
disallow = all
;allow = slin
allow = ulaw
allow = adpcm
allow = gsm
transfer = no
[iaxclient] ; Connect from iax client (Zoiper...)
type = friend ; Notice type here is friend <--------------
context = iax-client ; Context to jump to in extensions.conf
auth = md5
secret = af8e76d867961118710cb9e125f1ad29
host = dynamic
disallow = all
allow = ulaw
allow = adpcm
allow = gsm
transfer = no
[allstar-sys]
type = user
context = allstar-sys
auth = rsa
inkeys = allstar
disallow = all
allow = ulaw
allow = adpcm
allow = gsm
[allstar-public]
type = user
context = allstar-public
auth = md5
secret = allstar
disallow = all
;calltokenoptional=0.0.0.0/0.0.0.0
requirecalltoken=no
allow = ulaw
allow = adpcm
allow = gsm
I have to admit that "I" continue to get confused with Asterisk contexts and the dialplan. So, I posed a few questions to my favorite AI 
If you recall, exec'ing rpt lookup 12345 in the Asterisk CLI returns something like :
Node: 12345 Data: radio@127.0.0.1/12345,NONE
The "radio" would be the context in the iax.conf file.
Now, within the [radio] context in iax.conf you have context = radio-secure and that would be the context in extensions.conf ... and that's why the extension logging are saying @radio-secure.
As for the codec priority, if you have a server/node where you prefer one codec over another then don't forget to specify codecpriority=host.
and, hopefully, this helps to clear up some of the confusion.