Update firewall_devices.rst

source/firewall/firewall_devices.rst

@@ -7,9 +7,28 @@ Firewall device settings that help with SIP connections.
 
 
 
-**SIP ALG-**
+**SIP ALG**
 ^^^^^^^^^^^^^^^
 Most of the time this setting is set to off or disabled and varies.  Rarely this should be enabled. Below is a list a devices that need setting changes to address SIP ALG issues.  
 
-* Zyxel
+* `Zyxel`_
 
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+.. _Call Block: http://docs.fusionpbx.com/en/latest/firewall/firewall_devices/zyxel_sip_alg.html