Blocking typical traffic in and out that I would do for a pbx in America.
just use a strict firewall policy. do not put a permit any to any rule in there. only permit what is needed. are you hosting a website or any other services?
Using Snort to try and determine possible intrusions.
there is a package in pfsense for this. install it and take a look.
Using very complicated passwords on all AP's (including hidden SSID, password with random spaces in it, non-sensical SSID if discovered, and MAC filtering)as well as non-descript computer names, network drives, etc,
hidden SSID's and mac filtering isn't going to buy you much if any at all. security by obscurity is a very bad practice. ssid's can still be sniffed and mac filtering is easily spoofed. what you need to be sure is that your using the strongest encryption available. you need WPA2 with AES. anything less is vulnerable.