In some case when using an IAX extension, you have to define extension specific settings to make it work properly. For instance, in one of my server I had to add the following three lines:
encryption=aes128
auth=md5
username=myname
There is no way to add these settings in the FreePBX page, and you can't really add it manually in the iax_additional.conf file as it will be rewritten at the next change.
I would suggest either to add at least these three specific fields in the iax extension page, or to add a textbox to add free text to be included in the iax extension settings. I'm sure that they are some other settings pertaining to calltoken that should also be interesting to be able to enter on a per extension basis instead that in an iax_general_custom.conf