You should only require a single context and being a trunk, you should be able to set it as a peer (vs. a friend).
There should not be anything that changed at that level going to 13. If you check your sip_additional.conf file from a backup and that generated on 13, for the given context, they should be the same.
If you changed Asterisk versions in the process of your upgrades then it's possible (though unlikely for this scenario) that something changed.
Also, if your calls are not coming in from inbound18.viteily.net from them, and some overall SIP defautlts changed as a result of the upgrade (that is possible) then it could be that the calls were previously coming in as 'unknown' but you were letting them in, and the new settings are not. This is best fixed by having a proper trunk configuration (if it's not), vs. allowing unknown SIP INVITE's into the system (which you could do form the SIP Settings module. So, that's something worth checking.
So ... my best guess is it's one of these issues and solvable by a more 'proper' configuration on your end. But these are all things that you can check and verify. I say should because it's always possible that a bug was introduced that is doing something different in 13. But given how many upgrades there have been without an issue, I'm suggesting you look at these a bit closer to see if it's more in your control.