[gentoo-user] public wifi blocking ports

[Joseph]

I've installed "zoiper" (this is an softphone app to connect to my Asterisk server) on my old phone and it works on my private network over wifi.
I'm using standard IAX port 4569 to register, so this port is open on my firewall.

But when I catch an open public wifi network in a Mall or a Tim Horton "zoiper" failed to register.

Do they block outgoing ports of public WiFi networks? 
What are my alternatives?

I can open any port on my DD-Wrt and redirect it to my Asterisk server.

-- 
Joseph