SIP traffic pass over firewall even if i set a deny rule

Hello,

I have a ZyXEL USG 2000 software version: 3.30(AQW.7)

Behind i have a sip server (asterisk) and i wonder how it is possible for sip traffic to pass the firewall regardless of the blocking rules I set on the router.

Does this router have a special and invisible rule in the firewall regarding sip traffic?

If i block the sip server(linux ip extern) from zyxel router firewall i can't connect to any service to that linux (http,ssh,..), but the phones can connect to the sip server…i don t know why

If i allow sip server (linux) from router firewall everything is working (ssh, http,sip,…)

And the firewall rule is simple…just the ip address (not http service or ssh service)

10x