We use a mix of Adtrans behind public IPs as well as those on private IPs (behind other public routers running NAT). There are no issues with Adtran CPEs in either topology.
Most ISPs have issues when firewalls in front of SIP endpoints are SIP-aware (are running SIP ALG, for instance), as those firewalls tend to modify SIP packets in ways that may not be best for ISP purposes. While we don't deploy any CPEs behind firewalls, as long as the firewall is configured correctly (i.e. the SIP ALG feature is disabled) it should not cause any issues for the Adtran behind it. You could also use the firewall to perform simple NAT and use the Adtran's built-in firewall to further police the traffic.