If other methods (to maintain an open NAT route) don't succeed, you might try registering every 179 seconds. Even though the proxy will record the registration for 600 seconds and return "expires=600", it will accept another registration before the first has expired. A better solution would be to forward some ports in the router, but of course this is not always possible.
Maybe someday there will be a real NAT-keep-alive module written for Asterisk. A proper NAT-keep-alive module would allow a choice of time interval, and allow sending of empty packets which are better for NAT-keep-alive because they don't add load to the proxy by causing it to respond.
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