Asterisk server not behind NAT, but clients are


I am an Asterisk newbie trying to understand a few things. My Asterisk server has a public IP address and is not behind a firewall, nor is running iptables. For clients I am using ExpressTalk (OS X) behind a Juniper SRX210 firewall doing NAT and sipdroid on my Verizon Droid phone. Could someone please point me in a direction with this configuration - should I use a STUN server for client-to-client (ExpressTalk to sipdroid) calls? Do I need to configure an ALG on the SRX 210?

Thanks in advance!