This caused me sooo many hours of grief.
If you try to make SIP calls over the Internet, but your Asterisk box is behind a router, you may experience a problem where the call will connect but you won’t hear any voice. Here is what you have to do to fix it:
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Go to your rtp.conf file and note the values for rtpstart and rtpend
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On your router, add port forwarding for the SIP port (5060) and for the entire range of ports between rtpstart and rtpend.
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Find your EXTERNAL IP address by clicking this link: http://checkip.dyndns.org/
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Go to your sip.conf file and add the following under the [general] context:
nat=yes
externip=123.123.123.123 (replace with your external ip address)
localnet=10.0.0.0/255.0.0.0 (replace with your local network address and mask)
canreinvite=no
- restart asterisk
Good luck!