SIP ALG, allowguest=no

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Hello

I just read this article about an Asterisk server that got hacked to
make free international calls through an ITSP:

www.rowetel.com/blog/?p=2210

I have a couple of questions:

1. Am I correct in understanding that SIP ALG on a router makes it
easier to host an Asterisk server on a private LAN behind a NAT router
(no need to map ports for RTP + outgoing packets can be sent directly
to the remote SIP client instead of going through the Asterisk server
to rewrite the RTP port numbers)?
www.voip-info.org/wiki/view/Routers+SIP+ALG

2. If “allowguest=no” is commented out, it means that any SIP client
on the Net can connect to the Asterisk server and make outgoing calls
like legitimate SIP clients?

Thank you.