Overlap signalling.

Собственно, вот цитата из RFC 2543:

      The server received a request with a To (Section 6.37) address or
      Request-URI that was incomplete. Additional information SHOULD be
      provided.


           This status code allows overlapped dialing. With overlapped
           dialing, the client does not know the length of the dialing
           string. It sends strings of increasing lengths, prompting
           the user for more input, until it no longer receives a 484
           status response.


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