The ERROR-CODE attribute is used in error response messages. It
contains a numeric error code value in the range of 300 to 699 plus a
textual reason phrase encoded in UTF-8 RFC3629; it is also
consistent in its code assignments and semantics with SIP RFC3261
and HTTP RFC7231. The reason phrase is meant for diagnostic
purposes and can be anything appropriate for the error code.
Recommended reason phrases for the defined error codes are included
in the IANA registry for error codes. The reason phrase MUST be a
UTF-8-encoded RFC3629 sequence of fewer than 128 characters (which
can be as long as 509 bytes when encoding them or 763 bytes when
decoding them).