It will generate a **WWW-Authenticate** header field  containing a digest challenge (and not <del>WWW-Authorize</del> header field as written in the manual page).

The header field looks like this:
WWW-Authenticate: Digest realm="192.168.1.15", nonce="462791f06aa9b37a34fd11b1adc58c5a9df7c95b"

This challenge is transmitted in a 401 (Unauthorized) reply.

The UA sends its response within the second request attempt, which contains a header field looking as follows:
Authorization: Digest username="10", realm="192.168.1.15", algorithm=MD5, uri="sip:192.168.1.15", nonce="462791f06aa9b37a34fd11b1adc58c5a9df7c95b", response="be9e8d46b8dcd522f22301e1f2b3287c"

[Emin Gabrielyan, 070424]