Update firewall.rst (#543)

Better explanation of ALG

administrator-manual/en/firewall.rst

@@ -105,8 +105,15 @@ If hair-pinning is still required, check the :guilabel:`Enable hairpin NAT` opti
 Application Level Gateway (ALG)
 -------------------------------
 
-ALG allows SIP and H.323 protocols to operate through NAT. When enabled, ALG will inspect and rewrite Voice over Internet Protocol (VoIP) network packets and open required ports. 
-ALG is enabled by default, but if you're if you're experiencing audio and call problems with your PBX or VoIP client try to disable it.
+Application level gateway is a security component that augments a firewall or NAT employed in a computer network.
+ALG is enabled by default on |product|, it supports many application protocols (like FTP, SIP and so on), allowing them to operate through NAT.
+ALG inspects and rewrites specific network packets and automatically opens required ports.
+
+**Enable SIP-ALG**
+
+Some PBXs may not work properly with SIP-ALG and H.323-ALG, if you're experiencing audio and call problems with your PBX or your VoIP client try to disable them.
+To disable the SIP-ALG and H.323-ALG modules untick the checkbox.
+
 
 .. _firewall_mac_binding-section: