The firewall was never the issue (I’ve even tried several configurations both with and without the firewall and found that enabling/disabling the firewall changes nothing [if you’re talking about responsive SIP accept, it doesn’t work in this case apparently]).
Instead, what seems to be the issue between the UEs and FreePBX is an authentication header mismatch. Since the UE’s are cellphones which use SIM cards, the authentication header is meant to be static from the phone, which means it can’t adjust to FreePBX’s md5 response like a softclient (software based client) can. This means I either need to find a way to change the SIM card’s authentication parameters (which I don’t think is possible), or change/disable the authentication parameters of FreePBX for the UE extension in which the UE is trying to register on. This is why I was questioning on whether or not it was possible to modify the custom_*.config files to accomodate the static nature of the UE’s authentication header.