This has bugged me for far too long. I think at one point I did have this resolved but then I hosed my pbx and when I restored it I forgot what I had done. In any event, the problem for me was converting the Freepbx information to Wazo.
When you read the write-up by Ward Mundy it mentions that security is handled by registering your IP with telecomsxchange first. That only verifies the IP addresses that can access their system. Also, in the Freepbx list it doesn’t mention anything about usernames and passwords.
I kept getting a ‘Failed to authenticate on INVITE’ message in my logs. Lots of time wasted but now I have it resolved. Your Telecomsxchange profile includes a username and password. You do need to enter that in the SIP Trunk section of Wazo. From there the settings pretty much line up.
In the outgoing calls section is where you will use the Extern prefix box for your external prefix. I did put my “1” after the # in extern prefix as that is how it is listed on the TelecomsXchange site.