I’ve never worked on the 57i to know but I can say for sure that the 9133s do. I have one at home behind a iptables firewall and it works great. Two other employee’s are behind a Linksys and dlink routers without a problem.
I’d go directly to the REAL aastra support people instead of your supplier and ask. I’d be shocked if they produced a phone that would not allow it to work behind a firewall after having phones that do. the 5xi series is supposed to be everything the 91xx series is plus more and not doing NAT would be a serious step backwards.