[Asterisk-Users] security

[Altus Snyman]

Good day all
I want to put a asterisk server on a public ip and allow any,registered
sip and iax connection
What security risks are there and how can I secure my pabx
One thing I want to know is how do I make it that anyone can call a
extension at my box but not make a call out.
i.o.w how do I call 303 at 192.168.2.1 and how do I make it that it cant
call 0082356124562462578 at 192.168.2.1
Please help me with these question
Thanks
Altus