One of the most frustrating things about Asterisk is that it can't handle simple problems. These are problems that you would think have been solved by now. I'm talking about DNS issues and the SIP channel driver.
You would think that if your internet connection went down that you would only lose your VOIP trunks or remote extensions. Well think again! When you lose the internet or better yet DNS access your Asterisk SIP Channel driver comes to a halt. That means that your local SIP extensions will also not work. In fact a lot of things just seem to halt until your internet or DNS is functioning again.
Why is this happening? Certain things should time out! My local SIP extensions and local SIP gateway should still work. I expect the internet to go down - I expect DNS resolution failures. I don't expect that after all this time the SIP channel driver is still broken.
I've looked in the Asterisk mailing list and this has been pointed out many times before - nobody wants to fix this.
I have developed ways around this that don't require code changes but this is something that should be fixed at the root cause.