In IMS 4xx SIP responses are defined as customer failures but the doubt raised is whether they are always dependent to “customer behaviors”/“customer failures” or they can be also indirect subsequence to network failures. For Network failures, we know, a different set, 5xx, exists but looking for the reason behind each 4xx, especially for some of them (e.g. “403 forbidden” or “404 not found” or “480 temporary unavailable”), you live the doubt that they can be produced by a wrong network configuration. What is your opinion? Could I surely consider the 4xx family consequence of something wrong from customer side and excluding network errors accordingly?
In my experience, not only end user bad behaviour but more core related. Like intermidiate device betwen ran and IMS network. Check any outage or planned activity done during that time you receive that 4xx error code.
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