ref link; http://www.experts-exchange.com/Microsoft/Windows_Security/Microsoft_ISA/Q_23244135.html

When using Asterisk you will need to make sure the following ports are redirected to the asterisk server.

SIP - UDP 5060
RTP - UDP 10000-20000

Then within asterisk itself you need to define the l'localip' and either the 'externip' or 'externhost' within sip.conf. This is so that when asterisk detects a call coming from the internet it uses the external IP address within the SIP packets.

name: RTP
Parameters:

Primay connections:
- 8000-10000 - UDP - Recieve Send
Secondary connections:
- 8000-10000 - UDP - Send Receive

And here is my RTP.CONF
rtpstart=8000
rtpend=10000

And here my SIP.CONF
context=default
allowoverlap=no
bindport=5060
bindaddr=0.0.0.0
srvlookup=yes
externip=
nat=yes
localnet=192.168.30.0/255.255.255.0