Are you both behind firewalls that are uncontrollable? Or saying, at least, that we should assume that?

Ideally, I'm looking for a long-term solution that will work no matter who I'm communicating with. At the moment, I'm behind a firewall that I can control, but I can't always assume it'll be that way. The person at the other end of the line may often be behind a firewall they can't control. So yeah, for the purposes of this exercise, let's assume that one or both of the parties is behind an uncontrollable firewall.