SIP has loop detection and prevention built-in. However, when you are interfacing with legacy technology through a gateway there is a very real risk that a loop can be setup between the two systems. The scenario is a large migration from TDM to SIP. As you add users to the SIPX install, SIPX first looks at current registrations, then will route calls through the dial-plan. If through an accounting error your TDM systems sends calls to SIPX and there is no matching extension in SIPX then the call is routed back out to the TDM system, which routes it back into SIPX...you get the idea. This can result in calls that go above 40,000/hour! It will bring your proxies to their knees. Below is a simple script you can run from cron to block calls.
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